btrfs-progs: check for null string in parse_size

Because it's better than a segfault if it's called improperly,
and it makes static checkers happier.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>

utils.c

@@ -1382,7 +1382,7 @@ u64 parse_size(char *s)
 	char c;
 	u64 mult = 1;
 
-	for (i=0 ; s[i] && isdigit(s[i]) ; i++) ;
+	for (i = 0; s && s[i] && isdigit(s[i]); i++) ;
 	if (!i) {
 		fprintf(stderr, "ERROR: size value is empty\n");
 		exit(50);