lavf/avidec: Do not fail for very large idx1 tags.

Fixes demuxing the sample file from github pull request 197,
the size of its idx1 tag is 6171936 bytes, followed by a JUNK
tag of 9505704 bytes.

libavformat/avidec.c

@@ -993,6 +993,8 @@ FF_ENABLE_DEPRECATION_WARNINGS
                 avi->movi_end  = avi->fsize;
                 goto end_of_header;
             }
+        /* Do not fail for very large idx1 tags */
+        case MKTAG('i', 'd', 'x', '1'):
             /* skip tag */
             size += (size & 1);
             avio_skip(pb, size);