rtpdec_asf: fix memleak

Based on a suggestion by Ronald S. Bultje
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtpdec_asf.c

@@ -233,10 +233,14 @@ static int asfrtp_parse_packet(AVFormatContext *s, PayloadContext *asf,
 
                 int cur_len = start_off + len_off - off;
                 int prev_len = out_len;
+                void *newbuf;
                 out_len += cur_len;
-                asf->buf = av_realloc(asf->buf, out_len);
-                if(!asf->buf || FFMIN(cur_len, len - off)<0)
+                if(FFMIN(cur_len, len - off)<0)
                     return -1;
+                newbuf = av_realloc(asf->buf, out_len);
+                if(!newbuf)
+                    return -1;
+                asf->buf= newbuf;
                 memcpy(asf->buf + prev_len, buf + off,
                        FFMIN(cur_len, len - off));
                 avio_skip(pb, cur_len);