spellcasting level 0 monsters

Based on a report from <Someone>, avoid calling rn2(ml) when ml == 0.
Since level 0 monster always fail in their spells, just skip the whole
loop looking for a good spell.

doc/fixes34.1

@@ -125,6 +125,7 @@ prevent getting stuck in infinite loop when using wizard mode #levelchange
 naming an already wielded elven dagger "Sting" activates warning against orcs
 naming either of the wielded weapons unintentionally ends two-weapon combat
 Various nemesis monsters must resist stoning so their death messages make sense
+don't call DEBUG impossible in rn2 when a level 0 monster tries to cast a spell
 
 
 Platform- and/or Interface-Specific Fixes

src/mcastu.c

@@ -181,7 +181,7 @@ castmu(mtmp, mattk, thinks_it_foundyou, foundyou)
 	 * attacking casts spells only a small portion of the time that an
 	 * attacking monster does.
 	 */
-	if (mattk->adtyp == AD_SPEL || mattk->adtyp == AD_CLRC) {
+	if ((mattk->adtyp == AD_SPEL || mattk->adtyp == AD_CLRC) && ml) {
 	    int cnt = 40;
 
 	    do {