is_damageable was always true

<Someone> noticed that is_damageable used the address of is_rottable
instead of calling it.  Unfortunately, there doesn't seem to be a good
compiler warning to detect uses of function addresses as booleans.

doc/fixes34.2

@@ -35,6 +35,7 @@ dropping from height or throwing a normal container may damage contents
 some Magicbane messages treated "erinys" as plural
 initialize artifacts before processing $WIZKIT
 clean up inconsistency between various places quaff is documented
+is_damageable was using is_rottable incorrectly
 
 
 Platform- and/or Interface-Specific Fixes