[Fuchsia] Fix crash in FuchsiaDecryptor

FuchsiaDecryptor::CancelDecrypt() was trying to use |audio_decryptor_|
even if it wasn't created.

Bug: b/141148085
Change-Id: Ib015ba1739cb5b57a5b4174d2732e5ccc3568005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1842114
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703068}

media/fuchsia/cdm/fuchsia_decryptor.cc

@@ -40,7 +40,7 @@ void FuchsiaDecryptor::Decrypt(StreamType stream_type,
 }
 
 void FuchsiaDecryptor::CancelDecrypt(StreamType stream_type) {
-  if (stream_type == StreamType::kAudio) {
+  if (stream_type == StreamType::kAudio && audio_decryptor_) {
     audio_decryptor_->CancelDecrypt();
   }
 }