bug 2254 [1/3]: fixed GetAuthType(); avoid crash

bug 2254 [1/3]: fixed GetAuthType(); avoid crash

Improved C4::AuthoritiesMarc::GetAuthType() so that
it returns either a hashref (if the authority type exists)
or undef (if it does not exist).  The same
accessor should not be used to either return a single
value or all values of a settings list.  Note that
all existing clients of GetAuthType are expecting
either a single hashref or undef; none of them
expected the arrayref that could be returned by
the previous version of the accessor.

When BiblioAddsAuthorities is ON, addbiblio.pl
now checks the return value of GetAuthType and
no longer crashes as follows if the MARC framework
specifies an invalid authority type for a given subfield:

Can't coerce array into hash at .../cataloging/addbiblio.pl line 738.

No documentation changes.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>