moc: Fix parsing of the empty preprocessor command

When encountering a null preprocessing directive (which is supposed to
be ignored), the moc preprocessor will leave a PP_NEWLINE token in the
token stream. That will confuse the parser.
The PP_NEWLINE token need to be ignored in the preprocessing phase.

Task-number: QTBUG-22717
Change-Id: I1e502a7e5bc6fa8ce2f82109ba7199b95747ff0a
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>

src/tools/moc/preprocessor.cpp

@@ -915,6 +915,8 @@ void Preprocessor::preprocess(const QByteArray &filename, Symbols &preprocessed)
         case PP_ENDIF:
             until(PP_NEWLINE);
             continue;
+        case PP_NEWLINE:
+            continue;
         case SIGNALS:
         case SLOTS: {
             Symbol sym = symbol();

tests/auto/tools/moc/tst_moc.cpp

@@ -368,6 +368,13 @@ signals:
     void signalInIf3();
 #endif
 
+# //QTBUG-22717
+ # /*  */
+#
+
+ # \
+
+//
 public slots:
     void const slotWithSillyConst() {}