libavutil/mem: use size_t for the length in av_strdup()

the string length is not constrained to INT_MAX
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/mem.c

@@ -266,7 +266,7 @@ char *av_strdup(const char *s)
 {
     char *ptr = NULL;
     if (s) {
-        int len = strlen(s) + 1;
+        size_t len = strlen(s) + 1;
         ptr = av_realloc(NULL, len);
         if (ptr)
             memcpy(ptr, s, len);