rtpenc_aac: Fix calculation of the header size

Previously the high end byte was always set to zero. Also get
rid of an unnecessary multiplication (which in practice couldn't
overflow) before shifting.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpenc_aac.c

@@ -47,8 +47,8 @@ void ff_rtp_send_aac(AVFormatContext *s1, const uint8_t *buff, int size)
             memmove(p + 2, s->buf + 2, au_size);
         }
         /* Write the AU header size */
-        p[0] = ((au_size * 8) & 0xFF) >> 8;
-        p[1] = (au_size * 8) & 0xFF;
+        p[0] =  au_size >> 5;
+        p[1] = (au_size & 0x1F) << 3;
 
         ff_rtp_send_data(s1, p, s->buf_ptr - p, 1);