libavcodec/mlp_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
mlp parser cannot combine the frame, the poutbuf is not set.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mlp_parser.c

@@ -64,6 +64,7 @@ static int mlp_parse(AVCodecParserContext *s,
     s->key_frame = 0;
 
     *poutbuf_size = 0;
+    *poutbuf = NULL;
     if (buf_size == 0)
         return 0;