SkDeque: Move asserts prior to pointer dereference

If we popped too far, we want the assert mechanism to catch this
bug prior to us dereferencing a nullptr.

Change-Id: I681e530e8bcc15897bc52b3688d83f85a59d7449
Reviewed-on: https://skia-review.googlesource.com/c/191065
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed