Fix -Werror=sign-compare warning in zend_memnistr (#8042)

`error: comparison of integer expressions of different signedness: ‘size_t’ {aka
‘long unsigned int’} and ‘long int’ [-Werror=sign-compare]`

This is asserting `end >= haystack` as a precondition for callers (in debug
builds) and existing callers are correct.

An alternate option is to cast the left side to `int64_t`, but that may be
slightly inefficient for 32-bit builds.

Author: TysonAndre

Zend/zend_operators.h

@@ -939,15 +939,15 @@ zend_memnistr(const char *haystack, const char *needle, size_t needle_len, const
 		return haystack;
 	}
 
-	if (UNEXPECTED(needle_len > (end - haystack))) {
+	if (UNEXPECTED(needle_len > (size_t)(end - haystack))) {
 		return NULL;
 	}
 
 	const char first_lower = zend_tolower_ascii(*needle);
 	const char first_upper = zend_toupper_ascii(*needle);
 	const char *p_lower = (const char *)memchr(haystack, first_lower, end - haystack);
 	const char *p_upper = NULL;
-	if (first_lower != first_upper) { 
+	if (first_lower != first_upper) {
 		// If the needle length is 1 we don't need to look beyond p_lower as it is a guaranteed match
 		size_t upper_search_length = end - (needle_len == 1 && p_lower != NULL ? p_lower : haystack);
 		p_upper = (const char *)memchr(haystack, first_upper, upper_search_length);