Fixed bugreport:6200 Prevent server crash when releasing empty spellbooks.

git-svn-id: https://svn.code.sf.net/p/rathena/svn/trunk@16388 54d463be-8e91-2dee-dedb-b68131a5f0ec

src/map/skill.c

@@ -4105,6 +4105,9 @@ int skill_castend_damage_id (struct block_list* src, struct block_list *bl, int
 				for(i = SC_MAXSPELLBOOK; i >= SC_SPELLBOOK1; i--) // List all available spell to be released
 					if( sc->data[i] ) spell[s++] = i;
 
+                if ( i == 0 )
+                    break;
+
 				i = spell[s==1?0:rand()%s];// Random select of spell to be released.
 				if( s && sc->data[i] ){// Now extract the data from the preserved spell
 					skill_id = sc->data[i]->val1;