A shop-owned glob picked up by the hero was added to shop's bill
and if that shrank to nothing it moved from the unpaid portion to
used-up portion as intended. But once there it retained obj->owt
of 0 and if 'sanity_check' was enabled, that triggered a warning
every move until finally paid for. Both the 'Ix' list of used-up
items and itemized shop billing revealed a weight of 0 aum if
'wizweight' was enabled.
Keep track of the weight a glob had when it becomes unpaid, then
reset from 0 to that amount if it becomes used-up. This overloads
the obj->oextra->omid field which is an unsigned int previously
only used for corpses. Now for globs it is pre-bill obj->owt which
is also unsigned int. I didn't add new oextra access functions for
it; it is only used in two places and existing omid ones suffice.
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