regex: Add nullptr check in `_regex_free`, needed with PCRE2 10.42

Fixes #76174.

(cherry picked from commit 290db4a0b6c636b123297a8cbfa448ab6af76642)

modules/regex/regex.cpp

@@ -40,7 +40,9 @@ static void *_regex_malloc(PCRE2_SIZE size, void *user) {
 }
 
 static void _regex_free(void *ptr, void *user) {
-	memfree(ptr);
+	if (ptr) {
+		memfree(ptr);
+	}
 }
 
 int RegExMatch::_find(const Variant &p_name) const {