MINOR: hlua: fix return type for hlua_checkfunction() and hlua_checktable()

hlua_checktable() and hlua_checkfunction() both return the raw
value of luaL_ref() function call.
As luaL_ref() returns a signed int, both functions should return a signed
int as well to prevent any misuse of the returned reference value.
diff --git a/src/hlua.c b/src/hlua.c
index f567c8d..3550608 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -475,7 +475,7 @@
  * error if the argument is not a "function".
  * When no longer used, the ref must be released with hlua_unref()
  */
-__LJMP unsigned int hlua_checkfunction(lua_State *L, int argno)
+__LJMP int hlua_checkfunction(lua_State *L, int argno)
 {
 	if (!lua_isfunction(L, argno)) {
 		const char *msg = lua_pushfstring(L, "function expected, got %s", luaL_typename(L, argno));
@@ -490,7 +490,7 @@
  * error if the argument is not a "table".
  * When no longer used, the ref must be released with hlua_unref()
  */
-__LJMP unsigned int hlua_checktable(lua_State *L, int argno)
+__LJMP int hlua_checktable(lua_State *L, int argno)
 {
 	if (!lua_istable(L, argno)) {
 		const char *msg = lua_pushfstring(L, "table expected, got %s", luaL_typename(L, argno));