avformat/mlvdec: read_string() received unsigned size, make the argument unsigned

Fixes: infinite loop
Fixes: mlv-timeout-e3b8cab9835edecad6823baa057e029671329d04

Found-by: Paul Ch <paulcher@icloud.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 1e71cb2c8e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mlvdec.c

@@ -76,7 +76,7 @@ static int check_file_header(AVIOContext *pb, uint64_t guid)
     return 0;
 }
 
-static void read_string(AVFormatContext *avctx, AVIOContext *pb, const char *tag, int size)
+static void read_string(AVFormatContext *avctx, AVIOContext *pb, const char *tag, unsigned size)
 {
     char * value = av_malloc(size + 1);
     if (!value) {