8139751: Javac crash with -XDallowStringFolding=false

Summary: When string folding is disabled, need to keep the original expression.
Reviewed-by: mcimadamore

src/share/classes/com/sun/tools/javac/parser/JavacParser.java

@@ -980,7 +980,7 @@ public class JavacParser implements Parser {
          */
         protected JCExpression foldStrings(JCExpression tree) {
             if (!allowStringFolding)
-                return null;
+                return tree;
             ListBuffer<JCExpression> opStack = new ListBuffer<>();
             ListBuffer<JCLiteral> litBuf = new ListBuffer<>();
             boolean needsFolding = false;

test/tools/javac/parser/StringFoldingTest.java

@@ -23,7 +23,7 @@
 
 /*
  * @test
- * @bug 7068902
+ * @bug 7068902 8139751
  * @summary verify that string folding can be enabled or disabled
  */
 
@@ -85,11 +85,14 @@ public class StringFoldingTest {
 
         if (disableStringFolding) {
             if (text.contains("FOLDED")) {
-                throw new AssertionError("Expected string folding");
+                throw new AssertionError("Expected no string folding");
+            }
+            if (!text.contains("\"F\"")) {
+                throw new AssertionError("Expected content not found");
             }
         } else {
             if (!text.contains("FOLDED")) {
-                throw new AssertionError("Expected no string folding");
+                throw new AssertionError("Expected string folding");
             }
         }
     }