This was reported explicitly for originally being in face selectmode,
but could also crash when in vertex selectmode (when doing multiple
cuts).

The reason here is that MESH_OT_loopcut switches to edge select mode
(needed for TRANSFORM_OT_edge_slide, done in ringsel_finish), but was
only doing this on the active object's editmesh, all other participating
meshes would keep their selectmode which would now be out of sync with
both the active object's editmesh and scene settings for these.

This causes problems later in 'Select Linked'. Here, a mixture of
objects are used. First the viewcontext is set up with the active
object, then all participating objects are iterated (changing the
viewcontext to another object), then unified_findnearest would use that
changed viewcontext which would now contain the last object iterated. To
repeat: this could now have a different selectmode than the active
object which is later again used to get the nearest BMElem from in
EDBM_elem_from_selectmode. So in the failing case, we could get an
edge (but no face because of edge selectmode) from unified_findnearest,
EDBM_elem_from_selectmode would return NULL though (edge provided, but
in face selectmode), leading to the crash.

To solve this I assume it is best to change selectmode on all
participating meshes in multi-object editmode loopcut if necessary so
these are always in sync for following operations.
Alternatively, Select Linked (and probably lots more operators) would
have to be tweaked to pay closer attention which object is really used
to get selectmode from.

Note the selectmode is actually set back from edge selectmode in certain
cases (see USE_LOOPSLIDE_HACK), this patch changes that as well to act
on all participating meshes.