Use NGX_CONF_OK in some function return checks.

Use NGX_CONF_OK in some function return checks.

The functions ngx_http_merge_types() & ngx_conf_merge_path_value()
return either NGX_CONF_OK aka NULL aka ((void *)0) (probably) or
NGX_CONF_ERROR aka ((void *)-1).

They don't return an integer constant which is what NGX_OK aka (0) is.

Lets use the right thing in the function return check.

This was found with -Wzero-as-null-pointer-constant which was enabled
for C in GCC 15 (not enabled with Wall or Wextra... yet).

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>