avformat/sapdec: check av_strdup() return value and fix memleak

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/sapdec.c

@@ -142,6 +142,10 @@ static int sap_read_header(AVFormatContext *s)
         }
 
         sap->sdp = av_strdup(&recvbuf[pos]);
+        if (!sap->sdp) {
+            ret = AVERROR(ENOMEM);
+            goto fail;
+        }
         break;
     }