dvbsuboverlay: Handle non_modifying_colour_flag correctly in the RLE handlers

dvbsuboverlay: Handle non_modifying_colour_flag correctly in the RLE handlers

The check for when to not memset was checking on an undeterministic 'bits' variable value,
which is only meant to be used inside the loop earlier when it is supposed to check if
clut_index is 1 together with non_mod set, as per spec:

"non_modifying_colour_flag:  If set to '1' this indicates that the CLUT entry value '1'
is a non modifying colour. When the non modifying colour is assigned to an object pixel,
then the pixel of the underlying region background or object shall not be modified. This
can be used to create "transparent holes" in objects."

https://bugzilla.gnome.org/show_bug.cgi?id=666352