avcodec/flashsvenc: Change check for first frame

AVCodecContext.frame_number is actually only incremented
in case encoding was successfull; if e.g. the ff_alloc_packet()
below fails, it won't be incremented and therefore it is possible
for the previous_frame buffer to be allocated for multiple
first frames, leaking every one except the last.
So check for whether there already is a previous frame instead.

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

libavcodec/flashsvenc.c

@@ -209,7 +209,7 @@ static int flashsv_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     int opt_w = 4, opt_h = 4;
 
     /* First frame needs to be a keyframe */
-    if (avctx->frame_number == 0) {
+    if (!s->previous_frame) {
         s->previous_frame = av_mallocz(FFABS(p->linesize[0]) * s->image_height);
         if (!s->previous_frame) {
             av_log(avctx, AV_LOG_ERROR, "Memory allocation failed.\n");