Avoid throwing an error when there is no cached proxy object

[mayorova]

In case the API call doesn't pass thought the access phase (typical case would be if it is an /authorize or /oauth/token, or if the service is not found, so the exit is done in the rewrite phase), an error will be thrown in post_action:

2017/03/15 05:09:43 [error] 31748#0: *2 lua entry thread aborted: runtime error: .../apicast.lua:70: attempt to index local 'p' (a nil value)

Not sure if we also need to print some warning in the log in this case.

[octobot]

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[mikz]

This is fixed in #290 and thanks for explaining when it happens.
I'll write a test case for it.

[mikz]

Regression test pushed as ca45731