mind flayer's damage

From a followup to #H2247, April 2011...  Physical damage from a mind
flayer attack was inflicted in the AD_DRIN case for hitmu(), then
being inflicted again in the common code after the switch statement.
The new comment explaining the reason for non-standard damage is just
a guess.

doc/fixes36.1

@@ -124,6 +124,7 @@ override MSGTYPE=norep while executing the ':' command
 if a chameleon took vampire form, it would stop periodically changing shape
 corpses obtained from tipping an ice box wouldn't rot away
 suppress "you climb up the stairs" message if verbose option is off
+physical damage from mind flayer attack was being inflicted twice
 
 
 Platform- and/or Interface-Specific Fixes

src/mhitu.c

@@ -1091,9 +1091,11 @@ register struct attack *mattk;
                  helm_simple_name(uarmh));
             break;
         }
+        /* negative armor class doesn't reduce this damage */
         if (Half_physical_damage)
             dmg = (dmg + 1) / 2;
         mdamageu(mtmp, dmg);
+        dmg = 0; /* don't inflict a second dose below */
 
         if (!uarmh || uarmh->otyp != DUNCE_CAP) {
             /* eat_brains() will miss if target is mindless (won't