Bug with using revoke_tokens_on_logout = true

[arcivanov]

Turning on revoke_tokens_on_logout produces the following warning in the logs.

_G write guard:12: __newindex(): writing a global lua variable ('err') which may lead to race conditions between concurrent requests, so prefer the use of 'local' variables
stack traceback:
	/usr/local/share/lua/5.1/resty/openidc.lua:1171: in function 'openidc_revoke_token'
	/usr/local/share/lua/5.1/resty/openidc.lua:1213: in function 'openidc_logout'
	/usr/local/share/lua/5.1/resty/openidc.lua:1381: in function 'authenticate'

PR to follow