Fix potential null pointer dereference.

Fix potential null pointer dereference.

When the compiler does not perform return value optimization, the
LockPtr<> destructor of the temporary object is called after the move
constructor has set the Lock to null, thus causing a null pointer
dereference in the destructor. This can be replicated using the
-fno-elide-constructors build flag.

Change-Id: Ie00c3f93364fdf78ea1993469b9a606b3c87ebdc
Reviewed-on: https://chromium-review.googlesource.com/486985
Reviewed-by: Jim Stichnoth <stichnot@chromium.org>