pgsql: Improve handling of pthread_mutex_lock error case

Improve handling of pthread_mutex_lock error case

We should really be reporting a useful error along with returning
a valid return code if pthread_mutex_lock() throws an error for
some reason.  Add that and back-patch to 9.0 as the prior patch.

Pointed out by Alvaro Herrera

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/820739cba95622033527f60467a264db0ee91f76

Modified Files
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src/interfaces/libpq/fe-secure.c |   14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)