GDScript: Fix crash when base of an attribute is invalid

In attribute expressions (`a.b`) it's possible that the base has an
incorrect syntax and thus become a nullptr expression in the tree. This
commit add the check for this case to fail gracefully instead of
crashing.

modules/gdscript/gdscript_analyzer.cpp

@@ -2065,6 +2065,12 @@ void GDScriptAnalyzer::reduce_call(GDScriptParser::CallNode *p_call, bool is_awa
 		is_self = true;
 	} else if (callee_type == GDScriptParser::Node::SUBSCRIPT) {
 		GDScriptParser::SubscriptNode *subscript = static_cast<GDScriptParser::SubscriptNode *>(p_call->callee);
+		if (subscript->base == nullptr) {
+			// Invalid syntax, error already set on parser.
+			p_call->set_datatype(call_type);
+			mark_node_unsafe(p_call);
+			return;
+		}
 		if (!subscript->is_attribute) {
 			// Invalid call. Error already sent in parser.
 			// TODO: Could check if Callable here.

modules/gdscript/gdscript_parser.cpp

@@ -2545,7 +2545,7 @@ GDScriptParser::ExpressionNode *GDScriptParser::parse_attribute(ExpressionNode *
 
 	if (for_completion) {
 		bool is_builtin = false;
-		if (p_previous_operand->type == Node::IDENTIFIER) {
+		if (p_previous_operand && p_previous_operand->type == Node::IDENTIFIER) {
 			const IdentifierNode *id = static_cast<const IdentifierNode *>(p_previous_operand);
 			Variant::Type builtin_type = get_builtin_type(id->name);
 			if (builtin_type < Variant::VARIANT_MAX) {