avcodec/ac3enc: Use actual size of buffer in init_put_bits()

Since the very beginning (since de6d9b6404)
the AC-3 encoder used AC3_MAX_CODED_FRAME_SIZE (namely 3840) for the
size of the output buffer (without any check at all).
This causes problems when encoding EAC-3 for which the maximum is too small,
smaller than the actual size of the buffer: One can run into asserts used
by the PutBits API. Ticket #8513 is about such a case and this commit
fixes it by using the real size of the buffer.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/ac3.h

@@ -27,7 +27,6 @@
 #ifndef AVCODEC_AC3_H
 #define AVCODEC_AC3_H
 
-#define AC3_MAX_CODED_FRAME_SIZE 3840 /* in bytes */
 #define EAC3_MAX_CHANNELS 16          /**< maximum number of channels in EAC3 */
 #define AC3_MAX_CHANNELS 7            /**< maximum number of channels, including coupling channel */
 #define CPL_CH 0                      /**< coupling channel index */

libavcodec/ac3enc.c

@@ -1729,7 +1729,7 @@ static void ac3_output_frame(AC3EncodeContext *s, unsigned char *frame)
 {
     int blk;
 
-    init_put_bits(&s->pb, frame, AC3_MAX_CODED_FRAME_SIZE);
+    init_put_bits(&s->pb, frame, s->frame_size);
 
     s->output_frame_header(s);