h264: check sps.log2_max_frame_num for validity

Fixes infinite or long taking loop in frame num gap code in
the fuzzed sample bipbop234.ts_s223302.

CC: libav-stable@libav.org

libavcodec/h264_ps.c

@@ -37,6 +37,9 @@
 //#undef NDEBUG
 #include <assert.h>
 
+#define MAX_LOG2_MAX_FRAME_NUM    (12 + 4)
+#define MIN_LOG2_MAX_FRAME_NUM    4
+
 static const AVRational pixel_aspect[17]={
  {0, 1},
  {1, 1},
@@ -301,7 +304,7 @@ int ff_h264_decode_seq_parameter_set(H264Context *h){
     MpegEncContext * const s = &h->s;
     int profile_idc, level_idc, constraint_set_flags = 0;
     unsigned int sps_id;
-    int i;
+    int i, log2_max_frame_num_minus4;
     SPS *sps;
 
     profile_idc= get_bits(&s->gb, 8);
@@ -348,7 +351,16 @@ int ff_h264_decode_seq_parameter_set(H264Context *h){
         sps->bit_depth_chroma = 8;
     }
 
-    sps->log2_max_frame_num= get_ue_golomb(&s->gb) + 4;
+    log2_max_frame_num_minus4 = get_ue_golomb(&s->gb);
+    if (log2_max_frame_num_minus4 < MIN_LOG2_MAX_FRAME_NUM - 4 ||
+        log2_max_frame_num_minus4 > MAX_LOG2_MAX_FRAME_NUM - 4) {
+        av_log(h->s.avctx, AV_LOG_ERROR,
+               "log2_max_frame_num_minus4 out of range (0-12): %d\n",
+               log2_max_frame_num_minus4);
+        return AVERROR_INVALIDDATA;
+    }
+    sps->log2_max_frame_num = log2_max_frame_num_minus4 + 4;
+
     sps->poc_type= get_ue_golomb_31(&s->gb);
 
     if(sps->poc_type == 0){ //FIXME #define