Fix use-after-free with delayed interned memoized const operand

We should addref CONST operands during memoization, as they might
be destroyed by later compilation, e.g. through interning.

Author: nikic

Zend/tests/assign_coalesce_006.phpt

@@ -0,0 +1,12 @@
+--TEST--
+Null coalesce assign with memoized constant operand that is later interned (OSS-Fuzz #17903)
+--FILE--
+<?php
+$foo[__DIR__] ??= 42;
+var_dump($foo);
+?>
+--EXPECTF--
+array(1) {
+  ["%s"]=>
+  int(42)
+}

Zend/zend_compile.c

@@ -2054,6 +2054,9 @@ static void zend_compile_memoized_expr(znode *result, zend_ast *expr) /* {{{ */
 		} else if (result->op_type == IS_TMP_VAR) {
 			zend_emit_op_tmp(&memoized_result, ZEND_COPY_TMP, result, NULL);
 		} else {
+			if (result->op_type == IS_CONST) {
+				Z_TRY_ADDREF(result->u.constant);
+			}
 			memoized_result = *result;
 		}
 
@@ -7540,7 +7543,11 @@ void zend_compile_coalesce(znode *result, zend_ast *ast) /* {{{ */
 /* }}} */
 
 static void znode_dtor(zval *zv) {
-	efree(Z_PTR_P(zv));
+	znode *node = Z_PTR_P(zv);
+	if (node->op_type == IS_CONST) {
+		zval_ptr_dtor_nogc(&node->u.constant);
+	}
+	efree(node);
 }
 
 void zend_compile_assign_coalesce(znode *result, zend_ast *ast) /* {{{ */