fix #H2010 - quaffing wielded potion of polymorph

From a bug report, drinking a potion of
polymorph which is wielded would trigger an "object lost" panic if hero
took on a form that was forced to drop its weapon.  Once weapon/potion
got dropped, subsequent useup() of the potion was no longer operating on
an inventory object.

     Unwield the potion at start of drinking, so that dropping doesn't
come into play.  (If we ever introduce a monster form incapable of
holding inventory so drops everything, this will have to be revisited.)

doc/fixes34.4

@@ -384,6 +384,8 @@ hero poly'd into mimic and hiding as an object via #monster didn't unhide
 	when polymorphing into non-mimic
 message "You hit the  with all your might." could be issued if a boulder
 	went away while it was being dug/broken with a pick-axe
+prevent "object lost" panic if/when drinking a wielded potion of polymorph
+	causes hero's new form to drop weapon
 
 
 Platform- and/or Interface-Specific Fixes