Bug 1292505 (Followup) - Add an assert that we don't create a decoder for an ICO resource during a metadata decode. r=me

image/DecoderFactory.cpp

@@ -246,6 +246,7 @@ DecoderFactory::CreateDecoderForICOResource(DecoderType aType,
   MOZ_ASSERT(decoder);
 
   // Initialize the decoder, copying settings from @aICODecoder.
+  MOZ_ASSERT(!aICODecoder->IsMetadataDecode());
   decoder->SetMetadataDecode(aICODecoder->IsMetadataDecode());
   decoder->SetIterator(aSourceBuffer->Iterator());
   decoder->SetOutputSize(aICODecoder->OutputSize());