pgsql: The correct min buffer size is INITIAL_EXPBUFFER_SIZE, not

Log Message:
-----------
The correct min buffer size is
INITIAL_EXPBUFFER_SIZE, not PQERRORMSG_LENGTH.

Backpatch only, the proper fix in HEAD is
to use PQExpBuffers everywhere.

Tags:
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REL8_1_STABLE

Modified Files:
--------------
pgsql/src/interfaces/libpq:
fe-auth.c (r1.107.2.2 -> r1.107.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.107.2.2&r2=1.107.2.3)
fe-connect.c (r1.323.2.2 -> r1.323.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c.diff?r1=1.323.2.2&r2=1.323.2.3)
libpq-int.h (r1.108.2.2 -> r1.108.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-int.h.diff?r1=1.108.2.2&r2=1.108.2.3)