lib/stackdepot.c: allow the stack trace hash to be zero

Do not bail out from depot_save_stack() if the stack trace has zero hash.
Initially depot_save_stack() silently dropped stack traces with zero
hashes, however there's actually no point in reserving this zero value.

Change-Id: Ie289b59e70633657cafbb794fd05fbe5725ead28
Reported-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: 33334e25769c6ad69b983379578f42581d99a2f9
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Prakash Gupta <guptap@codeaurora.org>