Bug 16365 - GetMarcStructure() "unsafe" variant in PrepareItemrecordDisplay()

Bug 16365 - GetMarcStructure() "unsafe" variant in PrepareItemrecordDisplay()

This sub is a good candidate for the "unsafe" treatment too, it doesn't
modify nor autovivify anything in the marc structure.

Added a warning in the code regarding the $tagslib usage by the
custom item plugins, plus a small change to prevent possible
"Use of uninitialized value" warnings in the future.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested with neworderempty.pl and itemrecorddisplay.pl.
Amended slightly: Made warning less dramatic.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>