avformat/rtpdec_h264: Fix heap-buffer-overflow

Fixes: rtp_sdp/poc.sdp

Found-by: Bingchang <l.bing.chang.bc@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c42a1388a6)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rtpdec_h264.c

@@ -166,7 +166,7 @@ static int sdp_parse_fmtp_config_h264(AVFormatContext *s,
             parse_profile_level_id(s, h264_data, value);
     } else if (!strcmp(attr, "sprop-parameter-sets")) {
         int ret;
-        if (value[strlen(value) - 1] == ',') {
+        if (*value == 0 || value[strlen(value) - 1] == ',') {
             av_log(s, AV_LOG_WARNING, "Missing PPS in sprop-parameter-sets, ignoring\n");
             return 0;
         }