dbus-hash: Fix a potential shift by a negative integer

dbus-hash: Fix a potential shift by a negative integer

As a hash table becomes unbelievably large and full, the down_shift
tends towards 0. The overflow detection code in rebuild_table() does not
prevent down_shift becoming negative, which then causes undefined
behaviour in RANDOM_INDEX for int-keyed tables.

Note that this can only happen with approaching INT_MAX entries in the
hash table, at which point we’ve almost certainly hit OOM somewhere, so
this is vanishingly unlikely to happen. This is why I can’t add a test
for the bug.

As always, thanks to Coverity.

Coverity ID: 54682
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99641
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Change-Id: Iac3047fc0bff11b3d08c2938c0fda292bddb1466