Fixed bug with to-be-closed variables in base C level

To-be-closed variables in C use 'ci.nresults' to code that there is
a variable to be closed in that function. The intialization of the
base C level (the one "running" when calling API functions outside
any Lua call) did not initialize 'ci.nresults', creating (correct)
warnings in valgrind.

lstate.c

@@ -196,6 +196,8 @@ static void stack_init (lua_State *L1, lua_State *L) {
   ci->next = ci->previous = NULL;
   ci->callstatus = CIST_C;
   ci->func = L1->top;
+  ci->u.c.k = NULL;
+  ci->nresults = 0;
   setnilvalue(s2v(L1->top));  /* 'function' entry for this 'ci' */
   L1->top++;
   ci->top = L1->top + LUA_MINSTACK;