poly'd hero movement
Noticed while fixing the 'monster intrinsics from worn gear' bug(s):
set_uasmon() calls set_mon_data(&youmonst,...) which updates movement
when the monster polymorphs into something slower, then it did the
same thing to youmonst.movement itself, hitting the hero with a double
dose of reduction for any movement points not yet spent on current
turn. Remove the set_uasmon() side of that, and change set_mon_data()
side to add a redundant non-zero test to prevent static analysis from
warning that it might be dividing by 0.