samples/net: Fix error handling issues in IRC bot

Fix the following error:

Calling "transmit(irc->conn, pong, ret)" without checking return value.
It wraps a library function that may fail and return an error code.

Coverity-CID: 163115

Change-Id: I525e1059e6a2ae3896f68760f1a8a6c68ecfc074
Signed-off-by: Flavio Santes <flavio.santes@intel.com>