avcodec/webp: fix decoding for trailing junk

some bitstream have trailing junk, despite being valid webp data.
In case of apparent error, abort the loop and let *got_frame
decide whether this is an error or not.

fixes trac #8107 (/#7612)

Another possibility would be turning the loop into:
    while (!*got_frame) {...}
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/webp.c

@@ -1412,8 +1412,11 @@ static int webp_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
             return AVERROR_INVALIDDATA;
         chunk_size += chunk_size & 1;
 
-        if (bytestream2_get_bytes_left(&gb) < chunk_size)
-            return AVERROR_INVALIDDATA;
+        if (bytestream2_get_bytes_left(&gb) < chunk_size) {
+           /* we seem to be running out of data, but it could also be that the
+              bitstream has trailing junk leading to bogus chunk_size. */
+            break;
+        }
 
         switch (chunk_type) {
         case MKTAG('V', 'P', '8', ' '):