Avoid setting the stack top below upvalues to be closed

When leaving a scope, the new stack top should be set only after
closing any upvalue, to avoid manipulating values in an "invalid"
part of the stack.

lapi.c

@@ -171,19 +171,20 @@ LUA_API int lua_gettop (lua_State *L) {
 
 LUA_API void lua_settop (lua_State *L, int idx) {
   StkId func = L->ci->func;
+  int diff;  /* difference for new top */
   lua_lock(L);
   if (idx >= 0) {
-    StkId newtop = (func + 1) + idx;
-    api_check(L, idx <= L->stack_last - (func + 1), "new top too large");
-    while (L->top < newtop)
-      setnilvalue(s2v(L->top++));
-    L->top = newtop;
+    api_check(L, idx <= L->ci->top - (func + 1), "new top too large");
+    diff = (func + 1) + idx - L->top;
+    for (; diff > 0; diff--)
+      setnilvalue(s2v(L->top++));  /* clear new slots */
   }
   else {
     api_check(L, -(idx+1) <= (L->top - (func + 1)), "invalid new top");
-    L->top += idx+1;  /* 'subtract' index (index is negative) */
+    diff = idx + 1;  /* will "subtract" index (as it is negative) */
   }
-  luaF_close(L, L->top, LUA_OK);
+  luaF_close(L, L->top + diff, LUA_OK);
+  L->top += diff;  /* correct top only after closing any upvalue */
   lua_unlock(L);
 }