Editor: Do not assign unknown style to text style.

Just return the last style sentinel.

Change-Id: I2bd1a6e12913e0c208b11efae3baae87c183f39a
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/texteditor/texteditorconstants.cpp

@@ -110,7 +110,7 @@ TextStyle styleFromName(const char *name)
         if (qstrcmp(name, nameForStyle(TextStyle(i))) == 0)
             return TextStyle(i);
     }
-    return TextStyle();
+    return C_LAST_STYLE_SENTINEL;
 }
 
 } // namespace Constants