firmware-utils: uimage_padhdr: fix Coverity issue

firmware-utils: uimage_padhdr: fix Coverity issue

Fixes following issue reported by Coverity scan:

 *** CID 1452085:  Security best practices violations  (TOCTOU)
 /tools/firmware-utils/src/uimage_padhdr.c: 100 in main()
 94
 95     	if (!infname || !outfname) {
 96     		usage(argv[0]);
 97     		exit(1);
 98     	}
 99
 >>>     CID 1452085:  Security best practices violations  (TOCTOU)
 >>>     Calling function "stat" to perform check on "infname".
 100     	if (stat(infname, &statbuf) < 0) {

Fixes: a1c6a316d299 ("ramips: add support for Fon FON2601")
Signed-off-by: Petr Štetiar <ynezz@true.cz>