mux: Make sure that the data is actually written

And forward the error if it is not.

Bug-Id: 881

CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/mux.c

@@ -352,8 +352,12 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt)
     }
     ret = s->oformat->write_packet(s, pkt);
 
-    if (s->pb && ret >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)
-        avio_flush(s->pb);
+    if (s->pb && ret >= 0) {
+        if (s->flags & AVFMT_FLAG_FLUSH_PACKETS)
+            avio_flush(s->pb);
+        if (s->pb->error < 0)
+            ret = s->pb->error;
+    }
 
     return ret;
 }