scripts/gdb: fix list_for_each

scripts/gdb: fix list_for_each

If the list is uninitialized (next pointer is NULL), list_for_each gets
stuck in an infinite loop. Print a message and treat list as empty.

Link: https://lkml.kernel.org/r/4ae23bb1-c333-f669-da2d-fa35c4f49018@amazon.com
Signed-off-by: George Prekas <prekageo@amazon.com>
Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>