Fix up commit 4b3347, where I failed to notice that the initial
problem was really about the OID being interpreted as an integer
literal upon input, and overflowing its integer space before even making
it into pg_try_advisory_lock(). (We do still need to add -2147483648 to
make the result fit into an integer, as 4b3347 does.)

Hopefully fixes issue #30, for real this time.