res_odbc: Show only when there a fail attempt of connection in CLI

When is executed CLI command "odbc show all" every time is show
information about variable last_negative_connect. If not there  a fail
attempt of connection will show date like "1969-12-31 21:00:00".

This patch fix there situation for to show only this information when
exists a fail attempt before.

Change-Id: I7c058b0be6f7642e922de75ee6b82c7276c9f113