lua_getinfo always push a table (use L option) on stack even if it returns 0 (on error)

[云风 Cloud Wu]

I found when I call lua_getinfo with L option, it always push a table on stack, even if it returns 0 (on error).

Maybe writing this behavior into the document [1] would be better.

[1] http://www.lua.org/manual/5.3/manual.html#lua_getinfo