Handle missing class when evaluating CONST_ENUM_INIT

When resolving constants on a dynamically declared class during
preloading, the enum class may not be available. Fail gracefully
in that case.

Possibly we shouldn't be trying to evaluate constants on
non-linked classes at all?

Zend/zend_ast.c

@@ -782,6 +782,13 @@ ZEND_API zend_result ZEND_FASTCALL zend_ast_evaluate(zval *result, zend_ast *ast
 				: NULL;
 
 			zend_class_entry *ce = zend_lookup_class(class_name);
+			if (!ce) {
+				/* Class may not be available when resolving constants on a dynamically
+				 * declared enum during preloading. */
+				ZEND_ASSERT(CG(compiler_options) & ZEND_COMPILE_PRELOAD);
+				return FAILURE;
+			}
+
 			zend_enum_new(result, ce, case_name, case_value_zv);
 			break;
 		}

ext/opcache/tests/preload_enum.inc

@@ -10,3 +10,9 @@ class Test {
     public $x = MyEnum::Bar;
 }
 new Test;
+
+if (false) {
+    enum MyEnum2 {
+        case Foo;
+    }
+}

ext/opcache/tests/preload_enum.phpt

@@ -17,7 +17,9 @@ var_dump(MyEnum::Foo);
 var_dump(MyEnum::Bar);
 
 ?>
---EXPECT--
+--EXPECTF--
 enum(MyEnum::Bar)
+
+Warning: Can't preload unlinked class MyEnum2: Unknown reason in %s on line %d
 enum(MyEnum::Foo)
 enum(MyEnum::Bar)