hero health manipulation

I've been investigating issue #1252 (while the fuzzer was running,
sanity_check complained that hero's current health was greater
than maximum health) off and on for three months and haven't found
the cause.

I've checked all the places that lower maximum HP that I've managed
to find, but not spent much time looking for places that raise
current HP.

These changes might provide some more information.  They don't rely
on sanity_check being enabled.

Issue #1252 is still open.

src/mcastu.c

@@ -342,14 +342,12 @@ castmu(
         break;
     case AD_SPEL: /* wizard spell */
     case AD_CLRC: /* clerical spell */
-    {
         if (mattk->adtyp == AD_SPEL)
             cast_wizard_spell(mtmp, dmg, spellnum);
         else
             cast_cleric_spell(mtmp, dmg, spellnum);
         dmg = 0; /* done by the spell casting functions */
         break;
-    }
     } /* switch */
     if (dmg)
         mdamageu(mtmp, dmg);
@@ -441,6 +439,11 @@ death_inflicted_by(
 staticfn void
 cast_wizard_spell(struct monst *mtmp, int dmg, int spellnum)
 {
+    if (dmg < 0) {
+        impossible("monster cast wizard spell (%d) with negative dmg (%d)?",
+                   spellnum, dmg);
+        return;
+    }
     if (dmg == 0 && !is_undirected_spell(AD_SPEL, spellnum)) {
         impossible("cast directed wizard spell (%d) with dmg=0?", spellnum);
         return;
@@ -620,6 +623,12 @@ staticfn void
 cast_cleric_spell(struct monst *mtmp, int dmg, int spellnum)
 {
     int orig_dmg = 0;
+
+    if (dmg < 0) {
+        impossible("monster cast cleric spell (%d) with negative dmg (%d)?",
+                   spellnum, dmg);
+        return;
+    }
     if (dmg == 0 && !is_undirected_spell(AD_CLRC, spellnum)) {
         impossible("cast directed cleric spell (%d) with dmg=0?", spellnum);
         return;