Fix FETCH_OBJ_UNSET IS_UNDEF result

UNSET_OBJ et al. do not expect to find IS_UNDEF results for IS_INDIRECT vars. To
solve this, return IS_NULL from FETCH_OBJ_UNSET when properties are
uninitialized. Do the same for FETCH_OBJ_IS, as we're otherwise copying IS_UNDEF
into the VAR result, which is not a valid value for VAR.

Fixes OSS-Fuzz #429429090

Author: iluuu1994

Zend/tests/oss_fuzz_429429090.phpt

@@ -0,0 +1,20 @@
+--TEST--
+OSS-Fuzz #429429090
+--FILE--
+<?php
+
+class C {
+    public D $x;
+    static D $y;
+}
+
+$c = new C();
+isset($c->x[0]->prop);
+unset($c->x[0]->prop);
+isset(C::$y[0]->prop);
+unset(C::$y[0]->prop);
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/zend_execute.c

@@ -3533,6 +3533,9 @@ static zend_always_inline void zend_fetch_property_address(zval *result, zval *c
 	} else if (UNEXPECTED(Z_ISERROR_P(ptr))) {
 		ZVAL_ERROR(result);
 		goto end;
+	} else if (type == BP_VAR_UNSET && UNEXPECTED(Z_TYPE_P(ptr) == IS_UNDEF)) {
+		ZVAL_NULL(result);
+		goto end;
 	}
 
 	ZVAL_INDIRECT(result, ptr);
@@ -3684,6 +3687,11 @@ static zend_never_inline zval* zend_fetch_static_property_address_ex(zend_proper
 		return NULL;
 	}
 
+	if (UNEXPECTED(Z_TYPE_P(result) == IS_UNDEF)
+	 && (fetch_type == BP_VAR_IS || fetch_type == BP_VAR_UNSET)) {
+		return NULL;
+	 }
+
 	*prop_info = property_info;
 
 	if (EXPECTED(op1_type == IS_CONST)

Zend/zend_vm_def.h

@@ -1866,7 +1866,7 @@ ZEND_VM_INLINE_HELPER(zend_fetch_static_prop_helper, ANY, ANY, int type)
 		&prop_info, opline->extended_value & ~ZEND_FETCH_OBJ_FLAGS, type,
 		type == BP_VAR_W ? opline->extended_value : 0 OPLINE_CC EXECUTE_DATA_CC);
 	if (UNEXPECTED(!prop)) {
-		ZEND_ASSERT(EG(exception) || (type == BP_VAR_IS));
+		ZEND_ASSERT(EG(exception) || (type == BP_VAR_IS) || (type == BP_VAR_UNSET));
 		prop = &EG(uninitialized_zval);
 	} else if (UNEXPECTED(prop_info->flags & ZEND_ACC_PPP_SET_MASK)
 	 && (type == BP_VAR_W || type == BP_VAR_RW || type == BP_VAR_UNSET)