Fix memory leak in lexer token string allocation

The emit_token_with_str label was unconditionally allocating strings via
zend_copy_value, but these strings were only consumed when in parser mode
(i.e., when elem != NULL). When not in parser mode, the allocated strings
were never freed, causing memory leaks detected by LeakSanitizer.

Author: withinboredom

Zend/zend_language_scanner.l

@@ -3204,7 +3204,9 @@ nowdoc_scan_done:
 */
 
 emit_token_with_str:
-	zend_copy_value(zendlval, (yytext + offset), (yyleng - offset));
+	if (PARSER_MODE()) {
+		zend_copy_value(zendlval, (yytext + offset), (yyleng - offset));
+	}
 
 emit_token_with_val:
 	if (PARSER_MODE()) {