func_odbc: Check connection status before executing queries.

func_odbc: Check connection status before executing queries.

A recent change to func_odbc made it so that a single connection was
maintained per DSN. The problem was that the code was optimistic about
the health of the connection after initially opening it and did nothing
to re-connect in case the connection had died.

This change adds a check before executing a query to ensure that the
connection to the database is still up and running.

ASTERISK-25963 #close
Reported by Ross Beer

Change-Id: Id33c86eb04ff48ca088bb2e3086c27b3b683491d