Correct handling of format strings with escaped % specifications.

Note: It would be nice to be able to implement getformat() using
fmtcheck(3), but fmtcheck does not distinguish between signed and
unsigned types, a facility jot needs to perform range checks on its
output.

Submitted by:   Per Kristian Hove
MFC after:	2 weeks

usr.bin/jot/jot.c

@@ -393,8 +393,11 @@ getformat(void)
 	if (boring)				/* no need to bother */
 		return;
 	for (p = format; *p; p++)		/* look for '%' */
-		if (*p == '%' && *(p+1) != '%')	/* leave %% alone */
-			break;
+		if (*p == '%')
+			if (p[1] == '%')
+				p++;		/* leave %% alone */
+			else
+				break;
 	sz = sizeof(format) - strlen(format) - 1;
 	if (!*p && !chardata) {
 		if (snprintf(p, sz, "%%.%df", prec) >= (int)sz)