avutil/avstring: Use size_t in av_strlcatf()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit ae4eea8b)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/avstring.c

@@ -100,7 +100,7 @@ size_t av_strlcat(char *dst, const char *src, size_t size)
 
 size_t av_strlcatf(char *dst, size_t size, const char *fmt, ...)
 {
-    int len = strlen(dst);
+    size_t len = strlen(dst);
     va_list vl;
 
     va_start(vl, fmt);