TextEditor: Init markWidth variable

Otherwise some overrides of extraAreaWidth() may leave passed
value unmodified and we may end up with uninitialized value.

Change-Id: Ica89019fc2cff93f0bc7d784624730ec9d8c0224
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/texteditor.cpp

@@ -5886,7 +5886,7 @@ void TextEditorWidget::extraAreaMouseEvent(QMouseEvent *e)
 {
     QTextCursor cursor = cursorForPosition(QPoint(0, e->pos().y()));
 
-    int markWidth;
+    int markWidth = 0;
     extraAreaWidth(&markWidth);
     const bool inMarkArea = e->pos().x() <= markWidth && e->pos().x() >= 0;