libweston: fix realloc in weston_idalloc

realloc() does not initialize the added memory, it needs to be zeroed
explicitly.

Not zeroing this memory had one severe consequence: if the new
lowest_free_bucket was 0xffffffff, weston_idalloc_get_id() would fail an
assertion.

Otherwise non-zero bits in the added memory would just cause id numbers
to be skipped unnecessarily, which does no harm.

Fixes: #1000



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>