gdb: Remove redundant null check

This patch simplifies the code at two points by removing redundant
null checks.  There is no functional impact.

Reviewed-By: Keith Seitz <keiths@redhat.com>
Approved-By: Pedro Alves <pedro@palves.net>
Change-Id: I76e1c7fad00e8fcb24ced7bfd75d19cdd6266c32