roqaudioenc: Fix crash with very small roq files

Fixes CID703669
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/roqaudioenc.c

@@ -158,6 +158,8 @@ static int roq_dpcm_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
             context->input_frames++;
             return 0;
         }
+    }
+    if (context->input_frames < 8) {
         in = context->frame_buffer;
     }