From a bug report, it was possible to get
|You hit the with all your might. You stop digging.
if a boulder went away--in his case, it was picked up by a giant--while
you were occupied trying to break it with a pick-axe. The code explicitly
used "" to fill in the message when dig_target had an unexpected value.
This just avoids giving the message in a case like this. Possibly
extra stop_occupation() calls should be done instead, but I didn't want
to try to figure out how many would be needed (monster picks up object,
monster zaps wand of striking, others?).
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