Bug 12629 - Software error when trying to merge records from different frameworks

Bug 12629 - Software error when trying to merge records from different frameworks

Test Plan:
1) Choose a bib record that has the default framework.
2) Add a second record using that fast add framework.
3) Merge the records; switch to "Using framework: Default", and choose the original record (i.e. the one that had the default framework) as the merge reference. Clicking 'Next' will trigger the error.
4) Apply this patch
5) Repeat steps 1 - 3, no error should occur

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>