ffserver: initialize pbuffer in prepare_sdp_description()

also check pbuffer before use

Found-by: CSA
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 1d8d21b9)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffserver.c

@@ -2972,6 +2972,8 @@ static int prepare_sdp_description(FFStream *stream, uint8_t **pbuffer,
     AVOutputFormat *rtp_format = av_guess_format("rtp", NULL, NULL);
     int i;
 
+    *pbuffer = NULL;
+
     avc =  avformat_alloc_context();
     if (avc == NULL || !rtp_format) {
         return -1;
@@ -3008,7 +3010,7 @@ static int prepare_sdp_description(FFStream *stream, uint8_t **pbuffer,
     av_free(avc);
     av_free(avs);
 
-    return strlen(*pbuffer);
+    return *pbuffer ? strlen(*pbuffer) : AVERROR(ENOMEM);
 }
 
 static void rtsp_cmd_options(HTTPContext *c, const char *url)