pgsql: Add some code to ensure that we don't lose communication sync due

Log Message:
-----------
Add some code to ensure that we don't lose communication sync due to
an oversize message, per suggestion from Oliver Jowett.  I'm a bit
dubious that this is a real problem, since the client likely doesn't
have any more space available than the server, but it's not hard to
make it behave according to the protocol intention.

Modified Files:
--------------
    pgsql/src/backend/libpq:
        pqcomm.c (r1.172 -> r1.173)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c.diff?r1=1.172&r2=1.173)