Vault guards are more reluctant to turn up if they keep dying

This is primarily meant as a fix for a farming strategy, although
it also makes sense in terms of the decisions the guards would
likely make.

src/vault.c

@@ -320,12 +320,19 @@ invault(void)
     struct obj *otmp;
     boolean spotted;
     int trycount, vaultroom = (int) vault_occupied(u.urooms);
+    int vgdeathcount;
 
     if (!vaultroom) {
         u.uinvault = 0;
         return;
     }
-    ++u.uinvault;
+    /* after a couple of guards don't come back from their trips to
+       the vault, future guards become more reluctant to turn up (even
+       if summoned via whistle) */
+    vgdeathcount = svm.mvitals[PM_GUARD].died;
+    if (vgdeathcount < 2 ||
+        (vgdeathcount < 50 && !rn2(vgdeathcount * vgdeathcount)))
+        ++u.uinvault;
     if (u.uinvault < VAULT_GUARD_TIME
         || (u.uinvault % (VAULT_GUARD_TIME / 2)) != 0)
         return;
@@ -409,6 +416,10 @@ invault(void)
         EGD(guard)->vroom = vaultroom;
         EGD(guard)->warncnt = 0;
 
+        /* ensure the guard doesn't respawn again next turn if killed
+           immediately */
+        ++u.uinvault;
+
         reset_faint(); /* if fainted - wake up */
         /* if there are any boulders in the guard's way, destroy them;
            perhaps the guard knows a touch equivalent of force bolt;