fix #H7749 - "you detect it"

fix #H7749 - "you detect it"

'Detect' is used for observing a vampire shape change without being
able to see the vampire.  The problem here is that it changed from
bat form to fog cloud form in order to pass under a closed door,
and the message was being delivered when it was already at the door
location instead of before the move from a visible spot to that door.

I'm not happy with this fix, but any other alternative I considered
seemed to be worse.  Having the shape change use up the monster's
move is probably a better way to go.  Then on its next move it will
be in the right form to make a normal flow-under-door move.