Bug 1487792 - Do not drain the FFmpeg parsers. r=bryce

In FFmpeg 4.0 and later, draining the parser will cause later decoding error as the decoder expects to only be fed a null packet.
It was also unnecessary for earlier version of FFmpeg

Differential Revision: https://phabricator.services.mozilla.com/D5056

--HG--
extra : moz-landing-system : lando

dom/media/platforms/ffmpeg/FFmpegDataDecoder.cpp

@@ -155,11 +155,11 @@ FFmpegDataDecoder<LIBAV_VER>::DoDecode(MediaRawData* aSample, bool* aGotFrame,
 
   mLastInputDts = aSample->mTimecode;
 
-  if (mCodecParser) {
+  if (inputData && mCodecParser) { // inputData is null when draining.
     if (aGotFrame) {
       *aGotFrame = false;
     }
-    do {
+    while (inputSize) {
       uint8_t* data = inputData;
       int size = inputSize;
       int len = mLib->av_parser_parse2(
@@ -169,7 +169,7 @@ FFmpegDataDecoder<LIBAV_VER>::DoDecode(MediaRawData* aSample, bool* aGotFrame,
       if (size_t(len) > inputSize) {
         return NS_ERROR_DOM_MEDIA_DECODE_ERR;
       }
-      if (size || !inputSize) {
+      if (size) {
         bool gotFrame = false;
         MediaResult rv = DoDecode(aSample, data, size, &gotFrame, aResults);
         if (NS_FAILED(rv)) {
@@ -181,7 +181,7 @@ FFmpegDataDecoder<LIBAV_VER>::DoDecode(MediaRawData* aSample, bool* aGotFrame,
       }
       inputData += len;
       inputSize -= len;
-    } while (inputSize > 0);
+    }
     return NS_OK;
   }
   return DoDecode(aSample, inputData, inputSize, aGotFrame, aResults);