rtpenc_aac: Fix sending fragmented frames

After sending a fragmented frame, len (s->buf_ptr - s->buf) isn't
zero, while s->num_frames is zero as intended. Using s->num_frames
makes it work as intended, and is less convoluted than keeping track
of (resetting) s->buf_ptr.

This avoids sending stray data after sending a fragmented aac packet.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc_aac.c

@@ -39,7 +39,7 @@ void ff_rtp_send_aac(AVFormatContext *s1, const uint8_t *buff, int size)
 
     /* test if the packet must be sent */
     len = (s->buf_ptr - s->buf);
-    if ((s->num_frames == max_frames_per_packet) || (len && (len + size) > s->max_payload_size)) {
+    if ((s->num_frames == max_frames_per_packet) || (s->num_frames && (len + size) > s->max_payload_size)) {
         int au_size = s->num_frames * 2;
 
         p = s->buf + max_au_headers_size - au_size - 2;