sysprof: fix a message with stop without run

The function `misc.sysprof.stop()` reports that profiler is
already running:

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

both in `sysprof_error()` and fixes aforementioned problem.

Follows up tarantool/tarantool#781

src/lib_misc.c

@@ -335,6 +335,12 @@ LJLIB_CF(misc_sysprof_stop)
   return sysprof_error(L, PROFILE_ERRUSE, err_details);
 #else
   int status = luaM_sysprof_stop(L);
+  if (LJ_UNLIKELY(status == PROFILE_ERRRUN)) {
+      lua_pushnil(L);
+      lua_pushstring(L, err2msg(LJ_ERR_PROF_NOTRUNNING));
+      lua_pushinteger(L, EINVAL);
+      return 3;
+  }
   if (LJ_UNLIKELY(status != PROFILE_SUCCESS))
     return prof_error(L, status, NULL);
 

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

@@ -124,7 +124,8 @@ 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.