Fix bug in receiving requests in libcynara-agent

Fix bug in receiving requests in libcynara-agent

Method AgentSocketClient::receiveResponseFromServer should hang
until it can return a request received from cynara service.

However a single read from socket can read more than one request.
Received requests are queued.

Code did not check if there is a valid request read and queued
previously, but always tried to read from socket.

Fix changes order - so now code first checks queue and only in case,
when there is no valid request waiting a socket is read.

Change-Id: I845cd677700e516f252a8958b97ee8facb82170f