lua/flowvarlib: fix unchecked null

Use checkudata, instead of testudata which won't return in case of
NULL, but raise an error in the Lua script.

Fixes:

** CID 1646748:  Null pointer dereferences  (NULL_RETURNS)
/src/util-lua-flowvarlib.c: 89 in LuaFlowvarValue()

src/util-lua-flowvarlib.c

@@ -81,7 +81,7 @@ static int LuaFlowvarGet(lua_State *L)
 
 static int LuaFlowvarValue(lua_State *L)
 {
-    uint32_t *flowvar_id = luaL_testudata(L, 1, suricata_flowvar_mt);
+    uint32_t *flowvar_id = luaL_checkudata(L, 1, suricata_flowvar_mt);
     Flow *f = LuaStateGetFlow(L);
     if (f == NULL) {
         return LuaCallbackError(L, "flow is NULL");