libavcodec/png_parser.c: fix a use_of_uninitialized_value in target_dec_fuzzer.

the target_dec_fuzzer is checking for the avpkt.data pointer but if the
png parser cannot combine the frame, the poutbuf is not set.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/png_parser.c

@@ -45,6 +45,7 @@ static int png_parse(AVCodecParserContext *s, AVCodecContext *avctx,
     s->pict_type = AV_PICTURE_TYPE_NONE;
 
     *poutbuf_size = 0;
+    *poutbuf = NULL;
 
     if (!ppc->pc.frame_start_found) {
         uint64_t state64 = ppc->pc.state64;