sysprof: fix a message with stop without run

When sysprof is not started, the function `misc.sysprof.stop()`
reports that the profiler is already running:

| $ ./src/luajit -e 'print(misc.sysprof.stop())'
| nil     profiler is running already     22

The patch fixes that:

| $ ./src/luajit -e 'print(misc.sysprof.stop())'
| nil     profiler is not running         22

Follows up tarantool/tarantool#781

src/lib_misc.c

@@ -336,8 +336,14 @@ LJLIB_CF(misc_sysprof_stop)
   return prof_error(L, PROFILE_ERRUSE, err_details);
 #else
   int status = luaM_sysprof_stop(L);
-  if (LJ_UNLIKELY(status != PROFILE_SUCCESS))
+  if (LJ_UNLIKELY(status == PROFILE_ERRRUN)) {
+    lua_pushnil(L);
+    lua_pushstring(L, err2msg(LJ_ERR_PROF_NOTRUNNING));
+    lua_pushinteger(L, EINVAL);
+    return 3;
+  } else if (LJ_UNLIKELY(status != PROFILE_SUCCESS)) {
     return prof_error(L, status, NULL);
+  }
 
   lua_pushboolean(L, 1);
   return 1;

test/tarantool-tests/profilers/misclib-sysprof-lapi.test.lua

@@ -10,7 +10,7 @@ local test = tap.test("misclib-sysprof-lapi"):skipcond({
   ["Disabled due to #10803"] = os.getenv("LUAJIT_TEST_USE_VALGRIND"),
 })
 
-test:plan(43)
+test:plan(44)
 
 jit.off()
 -- XXX: Run JIT tuning functions in a safe frame to avoid errors
@@ -123,7 +123,8 @@ assert(res, err)
 
 -- Not running.
 res, err, errno = misc.sysprof.stop()
-test:ok(res == nil and err, "result status and error with not running")
+test:is(res, nil, "result status with not running")
+test:ok(err:match("profiler is not running"), "error with not running")
 test:ok(type(errno) == "number", "errno with not running")
 
 -- Bad path.