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 <[email protected]>