rtpenc_aac: Fix sending fragmented frames

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>