pgsql: Fix lo_read, lo_write, lo_truncate to cope with "size_t" length

Fix lo_read, lo_write, lo_truncate to cope with "size_t" length parameters.

libpq defines these functions as accepting "size_t" lengths ... but the
underlying backend functions expect signed int32 length parameters, and so
will misinterpret any value exceeding INT_MAX. Fix the libpq side to throw
error rather than possibly doing something unexpected.

This is a bug of long standing, but I doubt it's worth back-patching. The
problem is really pretty academic anyway with lo_read/lo_write, since any
caller expecting sane behavior would have to have provided a multi-gigabyte
buffer. It's slightly more pressing with lo_truncate, but still we haven't
supported large objects over 2GB until now.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0e924c007dbb74f8f7dbdb75810c9b9a8ed6d3ec

Modified Files
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doc/src/sgml/lobj.sgml | 56 ++++++++++++++++++++++++++++++++--------
src/interfaces/libpq/fe-lobj.c | 50 ++++++++++++++++++++++++++++++++---
2 files changed, 90 insertions(+), 16 deletions(-)