Qt4: do not explode if MyDocument isn't available

See http://forum.videolan.org/viewtopic.php?f=14&t=78396
(cherry picked from commit fe145872fccb389edc97c3dea2463f4956bc3db4)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/qt4.hpp

@@ -122,7 +122,7 @@ static inline QString QVLCUserDir( vlc_userdir_t type )
 {
     char *dir = config_GetUserDir( type );
     if( !dir )
-        abort();
+        return "";
     QString res = qfu( dir );
     free( dir );
     return res;