Use av_printf_format to check the usage of printf style functions

This helps catching cases where the format string doesn't
match what is passed in, or injection bugs where user data
is passed in as format string.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/internal.h

@@ -108,7 +108,7 @@ uint64_t ff_ntp_time(void);
  */
 int ff_url_join(char *str, int size, const char *proto,
                 const char *authorization, const char *hostname,
-                int port, const char *fmt, ...);
+                int port, const char *fmt, ...) av_printf_format(7, 8);
 
 /**
  * Append the media-specific SDP fragment for the media stream c