Handle invalid empty c++17 fold expression early

One of Qt sample snippets contain (...) as placeholder for actual user code.
Parsing that leads to invalid class cast because buildExpression would return
fold expression token marker and not a proper ICPPASTexpression.

Fix that by handling this error early as invalid fold expression.

core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/cxx17/FoldExpressionTests.java

@@ -164,13 +164,14 @@ public void testFoldExpressionRecognitionSuccess() throws Exception {
 	//      (... + 1 * vals);
 	//      (vals + ... + 1 * 2);
 	//      (1 * 2 + ... + vals);
+	//      (...);
 	//  }
 	public void testFoldExpressionErrors() throws Exception {
 		final String code = getAboveComment();
 		IASTTranslationUnit tu = parse(code, CPP, ScannerKind.STD, false);
 		ICPPASTTemplateDeclaration tdef = getDeclaration(tu, 0);
 		IASTFunctionDefinition fdef = (IASTFunctionDefinition) tdef.getDeclaration();
-		for (int i = 0; i < 13; ++i) {
+		for (int i = 0; i < 14; ++i) {
 			IASTProblemExpression e = getExpressionOfStatement(fdef, i);
 		}
 	}

core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/GNUCPPSourceParser.java

@@ -1287,7 +1287,7 @@ private int calculateFirstOffset(BinaryOperator leftChain, IASTInitializerClause
 
 	@Override
 	public final IASTExpression buildExpression(BinaryOperator leftChain, IASTInitializerClause expr) {
-		if (supportFoldExpression && leftChain != null && expr != null) {
+		if (supportFoldExpression && expr != null) {
 			int foldCount = 0;
 			int foldOpToken = 0;
 
@@ -1321,7 +1321,7 @@ public final IASTExpression buildExpression(BinaryOperator leftChain, IASTInitia
 
 			// Valid fold-expression has single ellipsis.
 
-			if (foldCount == 1) {
+			if (foldCount == 1 && leftChain != null) {
 				BinaryOperator rightChain;
 				if (foldOp == null) {
 					// unary right fold, remove expression and use left chain as is