Follow @eltank's advice re not being paranoid

There really shouldn't be any way that it'll return negative now, and
it would probably be better to have some chance of realizing this
(via the negative return) if it did.

src/monster.cpp

@@ -1772,7 +1772,7 @@ int monster::heal( const int delta_hp, bool overheal )
     if( hp > maxhp && !overheal ) {
         hp = maxhp;
     }
-    return std::max( 0, hp - old_hp );
+    return hp - old_hp;
 }
 
 void monster::set_hp( const int hp )