Fix crash if breakpoint commands contain detach or kill

Fix crash if breakpoint commands contain detach or kill

If breakpoint commands contain detach or kill, then gdb tries to access
freed memory:

(gdb) b main
Breakpoint 1 at 0x111d: file main.c, line 21.
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>detach
>end
(gdb) run
Starting program: /home/src/lappy/binutils-gdb.git/gdb/testsuite/gdb.base/main
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".

main () at main.c:21
21        return 0;
[Inferior 1 (process 241852) detached]
=================================================================
==241817==ERROR: AddressSanitizer: heap-use-after-free on address 0x7b7a3de0b760 at pc 0x55fcb92613fe bp 0x7ffec2d524f0 sp 0x7ffec2d524e0
READ of size 8 at 0x7b7a3de0b760 thread T0
    #0 0x55fcb92613fd in bpstat_do_actions_1 ../../gdb/breakpoint.c:4898
    #1 0x55fcb92617da in bpstat_do_actions() ../../gdb/breakpoint.c:5012
    #2 0x55fcba3180e7 in inferior_event_handler(inferior_event_type) ../../gdb/inf-loop.c:71
    #3 0x55fcba3ba1e1 in fetch_inferior_event() ../../gdb/infrun.c:4769

0x7b7a3de0b760 is located 0 bytes inside of 56-byte region [0x7b7a3de0b760,0x7b7a3de0b798)
freed by thread T0 here:
    #0 0x7f1a43522a2d in operator delete(void*, unsigned long) /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_new_delete.cpp:155
    #1 0x55fcb925d5cd in bpstat_clear(bpstat**) ../../gdb/breakpoint.c:4646
    #2 0x55fcbb69ea6a in clear_thread_inferior_resources ../../gdb/thread.c:185
    #3 0x55fcbb69f4cb in set_thread_exited(thread_info*, std::optional<unsigned long>, bool) ../../gdb/thread.c:244
    #4 0x55fcba368d64 in operator() ../../gdb/inferior.c:269
    #5 0x55fcba375e2b in clear_and_dispose<inferior::clear_thread_list()::<lambda(thread_info*)> > ../../gdb/../gdbsupport/intrusive_list.h:529
    #6 0x55fcba368f19 in inferior::clear_thread_list() ../../gdb/inferior.c:265
    #7 0x55fcba3694ba in exit_inferior(inferior*) ../../gdb/inferior.c:322
    #8 0x55fcba369e35 in detach_inferior(inferior*) ../../gdb/inferior.c:358
    #9 0x55fcba319d9f in inf_ptrace_target::detach_success(inferior*) ../../gdb/inf-ptrace.c:214
    #10 0x55fcba56a2f6 in linux_nat_target::detach(inferior*, int) ../../gdb/linux-nat.c:1582
    #11 0x55fcba62121c in thread_db_target::detach(inferior*, int) ../../gdb/linux-thread-db.c:1381
    #12 0x55fcbb5ca49e in target_detach(inferior*, int) ../../gdb/target.c:2557
    #13 0x55fcba356ba4 in detach_command(char const*, int) ../../gdb/infcmd.c:2894
    #14 0x55fcb9597eea in do_simple_func ../../gdb/cli/cli-decode.c:94
    #15 0x55fcb95b10b5 in cmd_func(cmd_list_element*, char const*, int) ../../gdb/cli/cli-decode.c:2831
    #16 0x55fcbb6f5282 in execute_command(char const*, int) ../../gdb/top.c:563
    #17 0x55fcb95eedb9 in execute_control_command_1 ../../gdb/cli/cli-script.c:526
    #18 0x55fcb95f04dd in execute_control_command(command_line*, int) ../../gdb/cli/cli-script.c:702
    #19 0x55fcb9261175 in bpstat_do_actions_1 ../../gdb/breakpoint.c:4940
    #20 0x55fcb92617da in bpstat_do_actions() ../../gdb/breakpoint.c:5012
    #21 0x55fcba3180e7 in inferior_event_handler(inferior_event_type) ../../gdb/inf-loop.c:71
    #22 0x55fcba3ba1e1 in fetch_inferior_event() ../../gdb/infrun.c:4769

previously allocated by thread T0 here:
    #0 0x7f1a435218cd in operator new(unsigned long) /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_new_delete.cpp:86
    #1 0x55fcb927061f in build_bpstat_chain(address_space const*, unsigned long, target_waitstatus const&) ../../gdb/breakpoint.c:5880
    #2 0x55fcba3d63b6 in handle_signal_stop ../../gdb/infrun.c:7083
    #3 0x55fcba3d01c7 in handle_inferior_event ../../gdb/infrun.c:6574
    #4 0x55fcba3b9918 in fetch_inferior_event() ../../gdb/infrun.c:4713

This checks after executing commands of each breakpoint if the bpstat
was deleted already, and stops any further processing immediately.
Now the result looks like this:

(gdb) b main
Breakpoint 1 at 0x111d: file main.c, line 21.
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>detach
>end
(gdb) run
Starting program: /home/src/lappy/binutils-gdb.git/gdb/testsuite/gdb.base/main
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".

main () at main.c:21
21        return 0;
[Inferior 1 (process 242940) detached]
(gdb)

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=14354
Approved-By: Andrew Burgess <aburgess@redhat.com>