pgsql: Improve handling of out-of-memory in libpq.

Improve handling of out-of-memory in libpq.

If an allocation fails in the main message handling loop, pqParseInput3
or pqParseInput2, it should not be treated as "not enough data available
yet". Otherwise libpq will wait indefinitely for more data to arrive from
the server, and gets stuck forever.

This isn't a complete fix - getParamDescriptions and getCopyStart still
have the same issue, but it's a step in the right direction.

Michael Paquier and me. Backpatch to all supported versions.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/fcdac561405e14e1ce9f8c07a53e7d4983afa7a0

Modified Files
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src/interfaces/libpq/fe-protocol2.c | 51 ++++++++++++++++++++--------
src/interfaces/libpq/fe-protocol3.c | 63 +++++++++++++++++++++++++----------
2 files changed, 83 insertions(+), 31 deletions(-)