monster hp loss with Stormbringer

<Someone> forwarded from rgrn that a monster using Stormbringer would
lose hp when hitting the player.  The sign of the "gain" was reversed due
to subtracting the difference in the wrong order.

doc/fixes34.3

@@ -2,6 +2,7 @@ $RCSfile$ $Revision$ $Date$
 
 General Fixes and Modified Features
 -----------------------------------
+monster draining the player using Stormbringer decreased monster's hitpoints
 
 
 Platform- and/or Interface-Specific Fixes

src/artifact.c

@@ -1157,7 +1157,7 @@ int dieroll; /* needed for Magicbane and vorpal blades */
 				      The(distant_name(otmp, xname)));
 			losexp("life drainage");
 			if (magr && magr->mhp < magr->mhpmax) {
-			    magr->mhp += (u.uhpmax - oldhpmax)/2;
+			    magr->mhp += (oldhpmax - u.uhpmax)/2;
 			    if (magr->mhp > magr->mhpmax) magr->mhp = magr->mhpmax;
 			}
 			return TRUE;