avcodec/mpeg4videodec: use av_fast_padded_malloc() for bitstream buffer

Fixes use of uninitialized memory with vlc (sample from ticket3277)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg4videodec.c

@@ -2653,10 +2653,9 @@ int ff_mpeg4_frame_end(AVCodecContext *avctx, const uint8_t *buf, int buf_size)
         }
 
         if (startcode_found) {
-            av_fast_malloc(&s->bitstream_buffer,
+            av_fast_padded_malloc(&s->bitstream_buffer,
                            &s->allocated_bitstream_buffer_size,
-                           buf_size - current_pos +
-                           FF_INPUT_BUFFER_PADDING_SIZE);
+                           buf_size - current_pos);
             if (!s->bitstream_buffer)
                 return AVERROR(ENOMEM);
             memcpy(s->bitstream_buffer, buf + current_pos,