pgsql: Fix pg_GSS_error to use conn->errorMessage more sanely, ie,

Log Message:
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Fix pg_GSS_error to use conn->errorMessage more sanely, ie, actually
work with the PQExpBuffer code instead of fighting it.  This avoids an
unnecessary limit on message length and fixes the latent bug that
errorMessage.len wasn't getting set.

Modified Files:
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    pgsql/src/interfaces/libpq:
        fe-auth.c (r1.136 -> r1.137)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c?r1=1.136&r2=1.137)