filter_lua: validate lua function

[nokute78]

filter_lua will be panic if call absent lua function.

how to reproduce

fluent-bit tries to call function no_such_func , but it is not defined.
fluent-bit will exit like this.

$ bin/fluent-bit -i dummy -F lua -p script=../scripts/test.lua -p call=no_such_func -m '*' -o null
Fluent-Bit v0.14.0
Copyright (C) Treasure Data

[2018/05/23 13:47:32] [ info] [engine] started (pid=28543)
PANIC: unprotected error in call to Lua API (attempt to call a nil value)

My patch is to validate lua function.
If it is invalid, fluent-bit ignores filter_lua and process other plugins.

$ bin/fluent-bit -i dummy -F lua -p script=../scripts/test.lua -p call=no_such_func -m '*' -o null
Fluent-Bit v0.14.0
Copyright (C) Treasure Data

[2018/05/23 13:45:02] [ info] [engine] started (pid=27909)
[2018/05/23 13:45:02] [error] [filter_lua] function no_such_func is not found
[2018/05/23 13:45:02] [error] Failed initialize filter lua.0

[edsiper]

thanks!