mpeg4video: ignore broken GOP headers

Some MPEG4 cameras produce files with empty GOP headers.
This patch makes the decoder ignore such broken headers and proceed
with the following I-frame.  Without this change, the following
start code is missed resulting in the entire I-frame being skipped.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/mpeg4videodec.c

@@ -1494,16 +1494,17 @@ end:
 
 static int mpeg4_decode_gop_header(MpegEncContext * s, GetBitContext *gb){
     int hours, minutes, seconds;
-
-    hours= get_bits(gb, 5);
-    minutes= get_bits(gb, 6);
-    skip_bits1(gb);
-    seconds= get_bits(gb, 6);
-
-    s->time_base= seconds + 60*(minutes + 60*hours);
-
-    skip_bits1(gb);
-    skip_bits1(gb);
+    unsigned time_code = show_bits(gb, 18);
+
+    if (time_code & 0x40) {     /* marker_bit */
+        hours   = time_code >> 13;
+        minutes = time_code >>  7 & 0x3f;
+        seconds = time_code       & 0x3f;
+        s->time_base = seconds + 60*(minutes + 60*hours);
+        skip_bits(gb, 20);      /* time_code, closed_gov, broken_link */
+    } else {
+        av_log(s->avctx, AV_LOG_WARNING, "GOP header missing marker_bit\n");
+    }
 
     return 0;
 }