pytdb: Fix repr segfault for internal db

pytdb: Fix repr segfault for internal db

The problem was tdb->name is NULL for TDB_INTERNAL databases, and
so it was crashing ...

    #0  0xb76944f3 in strlen () from /lib/i686/cmov/libc.so.6
    #1  0x0809862b in PyString_FromFormatV (format=0xb72b6a26 "Tdb('%s')", vargs=0xbfc26a94 "")
        at ../Objects/stringobject.c:211
    #2  0x08098888 in PyString_FromFormat (format=0xb72b6a26 "Tdb('%s')") at ../Objects/stringobject.c:358
    #3  0xb72b65f2 in tdb_object_repr (self=0xb759e060) at ./pytdb.c:439

Cc: 597089@bugs.debian.org
Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Jelmer Vernooij <jelmer@samba.org>