Editor: Fix crash in generic Highlighter for invalid rules

Change-Id: I3da9115ecd54f72256e4ee399b2cd37e31f6bcbe
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/texteditor/generichighlighter/specificrules.cpp

@@ -265,6 +265,9 @@ bool KeywordRule::doMatchSucceed(const QString &text,
                                  const int length,
                                  ProgressData *progress)
 {
+    if (m_list.isNull()) // Happens if a keyword rule points to a none existing keyword list
+        return false;
+
     int current = progress->offset();
 
     if (current > 0 && !definition()->isDelimiter(text.at(current - 1)))