Fix nil pointer dereference in container spec memory metrics

Fix nil pointer dereference in container spec memory metrics

Add nil checks for Memory.Limit, Memory.Reservation, and Memory.Swap
before dereferencing them in extractContainerSpecMetrics. These fields
are optional pointers in OCI runtime spec and can be nil when not set.

- Memory.Limit: Set from Pod resources.limits.memory
- Memory.Reservation: Only set via NRI plugins (rarely used)
- Memory.Swap: Set from memory_swap_limit_in_bytes (optional)

This prevents potential panic when collecting metrics from containers
that don't have all memory limits configured, and makes the code
consistent with CPU resource handling and helpers.go.

Signed-off-by: You Binhao <ymichaelson.0713@gmail.com>