lua/fiber: Fix abort on malformed join input

If I create a new fiber and the join to himself we get an abort:

 | tarantool> f = require('fiber')
 | ---
 | ...
 |
 | tarantool> f.join(f.self())
 | tarantool: src/fiber.c:407: fiber_join: Assertion `fiber->flags & FIBER_IS_JOINABLE' failed.
 | Aborted (core dumped)

we should better throw an error.

src/lua/fiber.c

@@ -676,10 +676,14 @@ lbox_fiber_join(struct lua_State *L)
 {
 	struct fiber *fiber = lbox_checkfiber(L, 1);
 	struct lua_State *child_L = fiber->storage.lua.stack;
-	fiber_join(fiber);
 	struct error *e = NULL;
 	int num_ret = 0;
 	int coro_ref = 0;
+
+	if (!(fiber->flags & FIBER_IS_JOINABLE))
+		luaL_error(L, "the fiber is not joinable");
+	fiber_join(fiber);
+
 	if (child_L != NULL) {
 		coro_ref = lua_tointeger(child_L, -1);
 		lua_pop(child_L, 1);

test/app/fiber.result

@@ -1454,6 +1454,14 @@ ch1:put(1)
 while f:status() ~= 'dead' do fiber.sleep(0.01) end
 ---
 ...
+--
+-- Test if fiber join() does not crash
+-- if unjoinable
+--
+fiber.join(fiber.self())
+---
+- error: the fiber is not joinable
+...
 -- cleanup
 test_run:cmd("clear filter")
 ---

test/app/fiber.test.lua

@@ -623,6 +623,12 @@ ch1:put(1)
 
 while f:status() ~= 'dead' do fiber.sleep(0.01) end
 
+--
+-- Test if fiber join() does not crash
+-- if unjoinable
+--
+fiber.join(fiber.self())
+
 -- cleanup
 test_run:cmd("clear filter")