Fixed bug for emergency collection in upvalue creation

When creating an upvalue, an emergency collection can collect the
previous upvalue where the new one would be linked. The following
code can trigger the bug, using valgrind on Lua compiled with the
-DHARDMEMTESTS option:

  local x; local y
  (function () return y end)();
  (function () return x end)()

lfunc.h

@@ -57,7 +57,7 @@ LUAI_FUNC Proto *luaF_newproto (lua_State *L);
 LUAI_FUNC CClosure *luaF_newCclosure (lua_State *L, int nelems);
 LUAI_FUNC LClosure *luaF_newLclosure (lua_State *L, int nelems);
 LUAI_FUNC void luaF_initupvals (lua_State *L, LClosure *cl);
-LUAI_FUNC UpVal *luaF_findupval (lua_State *L, StkId level);
+LUAI_FUNC void luaF_setupval (lua_State *L, StkId level, UpVal **slot);
 LUAI_FUNC void luaF_newtbcupval (lua_State *L, StkId level);
 LUAI_FUNC int luaF_close (lua_State *L, StkId level, int status);
 LUAI_FUNC void luaF_unlinkupval (UpVal *uv);