roqaudio: Always use the frame buffer on flush

Prevent NULL dereference.

CC: libav-stable@libav.org
Bug-Id: CID 703669
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
(cherry picked from commit 55b59fab)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/roqaudioenc.c

@@ -147,15 +147,16 @@ static int roq_dpcm_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
             context->input_frames++;
             return 0;
         }
-        in = context->frame_buffer;
     }
+    if (context->input_frames < 8)
+        in = context->frame_buffer;
 
     if (stereo) {
         context->lastSample[0] &= 0xFF00;
         context->lastSample[1] &= 0xFF00;
     }
 
-    if (context->input_frames == 7 || !in)
+    if (context->input_frames == 7)
         data_size = avctx->channels * context->buffered_samples;
     else
         data_size = avctx->channels * avctx->frame_size;