LP 115706: Avoid Internal Server Errors with Hold Count Retrieval

LP 115706: Avoid Internal Server Errors with Hold Count Retrieval

It can happen that the query to get a record's hold count can run too
long and timeout or fail for some other reason.  When this happens,
the user sees an Internal Server Error.  The underlying cause is the
following Perl error:

Can't use an undefined value as an ARRAY reference at
/usr/local/share/perl/5.22.1/OpenILS/Application/Circ/Holds.pm line
4264.

This commit rearranges the code around that line to avoid this
particular undefined value reference error.

Signed-off-by: Jason Stephenson <jason@sigio.com>