librpcsec_gss: Fix an off-by-one in rpc_gss_get_principal_name()

Include an extra byte for the nul-terminator, otherwise we may end up
with an out-of-bounds write.

The corresponding bug in the kernel implementation was fixed by commit
e3081f7e3e2d ("kgssapi(4): Fix string overrun in Kerberos principal construction").

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D57738