added comment and assert about an (impossible) division by zero

lgc.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lgc.c,v 2.196 2014/10/03 12:54:37 roberto Exp roberto $
+** $Id: lgc.c,v 2.197 2014/10/25 11:50:46 roberto Exp roberto $
 ** Garbage Collector
 ** See Copyright Notice in lua.h
 */
@@ -909,12 +909,15 @@ void luaC_checkfinalizer (lua_State *L, GCObject *o, Table *mt) {
 
 
 /*
-** set a reasonable "time" to wait before starting a new GC cycle;
-** cycle will start when memory use hits threshold
+** Set a reasonable "time" to wait before starting a new GC cycle; cycle
+** will start when memory use hits threshold. (Division by 'estimate'
+** should be OK: it cannot be zero (because Lua cannot even start with
+** less than PAUSEADJ bytes).
 */
 static void setpause (global_State *g) {
   l_mem threshold, debt;
   l_mem estimate = g->GCestimate / PAUSEADJ;  /* adjust 'estimate' */
+  lua_assert(estimate > 0);
   threshold = (g->gcpause < MAX_LMEM / estimate)  /* overflow? */
             ? estimate * g->gcpause  /* no overflow */
             : MAX_LMEM;  /* overflow; truncate to maximum */