Fix for Bug 6487 - No error explanation if patron expiration date is missing

Fix for Bug 6487 - No error explanation if patron expiration date is missing

The script tries to parse the patron expiration date without
checking that it is a valid date. If expiration date is empty
it doesn't get caught along with past expiration dates.

I wonder if this line was supposed to catch this problem:

Line 233: if ( $warning_year*$warning_month*$warning_day==0

This patch adds "!$borrower->{'dateexpiry'} ||" to that line.

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>