util: trim() fix for UB on pointer arithmetic

Follow-up fix on commit df4c061136

"
While it is true reversing the condition solves a single-byte read at
one before s, there is a second instance of UB.

Having a pointer to one before an object is in of itself UB in C, it's
on the side of language lawyering, but it's UB.
I add here a quote from a C standard draft:
> When an expression that has integer type is added to or subtracted
> from a pointer, the result has the type of the pointer operand.
> If both the pointer operand and the result point to elements of the
> same array object, or one past the last element of the array object,
> the evaluation shall not produce an overflow; otherwise, the
> behavior is undefined.
Taken from: http://www.iso-9899.info/n1570.html#6.5.6p8
"

Thanks Guilherme Janczak <guilherme.janczak@yandex.com>

util.c

@@ -60,8 +60,7 @@ static void
 trim(char *s) {
 	char *e;
 
-	e = s + strlen(s) - 1;
-	while(e > s && isspace((unsigned char)*e))
-		e--;
-	*(e + 1) = '\0';
+	for (e = s + strlen(s); e > s && isspace((unsigned char)*(e - 1)); e--)
+		;
+	*e = '\0';
 }