parse: language: Unsigned difference expression compared to zero

Squash CodeQL `cpp/unsigned-difference-expression-compared-zero`
issues.

    This rule finds relational comparisons between the result of
    an unsigned subtraction and the value `0`. Such comparisons
    are likely to be wrong as the value of an unsigned subtraction
    can never be negative. So the relational comparison ends up
    checking whether the result of the subtraction is equal to 0.
    This is probably not what the programmer intended.

Author: tlsa

src/parse/language.c

@@ -1194,15 +1194,15 @@ css_error parseNth(css_language *c,
 					data[consumed] != 'N'))
 				return CSS_INVALID;
 
-			if (len - (++consumed) > 0) {
+			if (++consumed < len) {
 				if (data[consumed] != '-')
 					return CSS_INVALID;
 
 				/* 2n- */
 				sign = -1;
 				had_sign = true;
 
-				if (len - (++consumed) > 0) {
+				if (++consumed < len) {
 					size_t bstart;
 
 					/* Reject additional sign */