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: a1c6a316d2 ("ramips: add support for Fon FON2601")
Signed-off-by: Petr Štetiar <ynezz@true.cz>

tools/firmware-utils/src/uimage_padhdr.c

@@ -97,18 +97,6 @@ int main(int argc, char *argv[])
 		exit(1);
 	}
 
-	if (stat(infname, &statbuf) < 0) {
-		fprintf(stderr,
-			"could not find input file. (errno = %d)\n", errno);
-		exit(1);
-	}
-
-	filebuf = malloc(statbuf.st_size + padsz);
-	if (!filebuf) {
-		fprintf(stderr, "buffer allocation failed\n");
-		exit(1);
-	}
-
 	ifd = open(infname, O_RDONLY);
 	if (ifd < 0) {
 		fprintf(stderr,
@@ -123,6 +111,18 @@ int main(int argc, char *argv[])
 		exit(1);
 	}
 
+	if (fstat(ifd, &statbuf) < 0) {
+		fprintf(stderr,
+			"could not fstat input file. (errno = %d)\n", errno);
+		exit(1);
+	}
+
+	filebuf = malloc(statbuf.st_size + padsz);
+	if (!filebuf) {
+		fprintf(stderr, "buffer allocation failed\n");
+		exit(1);
+	}
+
 	rsz = read(ifd, filebuf, sizeof(*imgh));
 	if (rsz != sizeof(*imgh)) {
 		fprintf(stderr,