avformat/hlsenc: Check that data is set

If codecpar->extradata is not set (for example, when the stream goes
through the 'tee' muxer), then a segfault occurs.
This patch ensures the data variable is not null before attempting
to access it
Before the var_stream_map option was available - I was using the tee
muxer to create each resolution as an individual stream.
When running this configuration after the most recent hlsenc change
I hit a segfault
The most simple command which recreates the segfault is:
ffmpeg -i in.ts -map 0:a -map 0:v -c:a aac -c:v h264 -f tee [select=\'a,v\':f=hls]tv_hls_hd.m3u8
Signed-off-by: Brendan McGrath <redmcg@redmandi.dyndns.org>

libavformat/hlsenc.c

@@ -308,7 +308,7 @@ static void write_codec_attr(AVStream *st, VariantStream *vs) {
 
     if (st->codecpar->codec_id == AV_CODEC_ID_H264) {
         uint8_t *data = st->codecpar->extradata;
-        if ((data[0] | data[1] | data[2]) == 0 && data[3] == 1 && (data[4] & 0x1F) == 7) {
+        if (data && (data[0] | data[1] | data[2]) == 0 && data[3] == 1 && (data[4] & 0x1F) == 7) {
             snprintf(attr, sizeof(attr),
                      "avc1.%02x%02x%02x", data[5], data[6], data[7]);
         } else {