Bug 1963 Problem with deleted biblio in a virtual shelf

Bug 1963 Problem with deleted biblio in a virtual shelf

This bug reappear in HEAD/3.8. When trying to display a virtual shelf
(list) containing a deleted biblio, this error message is displayed:

Software error:

Can't call method "field" on an undefined value at ....C4/Koha.pm line
1231.

This fix modify SQL query retrieving biblios, just skipping records from
virtual shelf table with no linked record in biblio table: LEFT JOIN
replace with JOIN.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>