pngdec: check if previous frame exists instead of trusting sequence_number

This fixes a segmentation fault caused by calling memcpy with NULL as
second argument in handle_p_frame_apng.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/pngdec.c

@@ -922,7 +922,8 @@ static int decode_fctl_chunk(AVCodecContext *avctx, PNGDecContext *s,
         return AVERROR_INVALIDDATA;
     }
 
-    if (sequence_number == 0 && dispose_op == APNG_DISPOSE_OP_PREVIOUS) {
+    if ((sequence_number == 0 || !s->previous_picture.f->data[0]) &&
+        dispose_op == APNG_DISPOSE_OP_PREVIOUS) {
         // No previous frame to revert to for the first frame
         // Spec says to just treat it as a APNG_DISPOSE_OP_BACKGROUND
         dispose_op = APNG_DISPOSE_OP_BACKGROUND;