pgsql: Make some marginal performance improvements in

Log Message:
-----------
Make some marginal performance improvements in reportErrorPosition(),
which turns out to be a dominant part of the runtime in scenarios
involving lots of parse-time warnings (such as Stephen Frost's example
of an INSERT with a lot of backslash-containing strings).  There's not
a whole lot we can do about the character-at-a-time scanning, but we
can at least avoid traversing the query twice.

Modified Files:
--------------
    pgsql/src/interfaces/libpq:
        fe-protocol3.c (r1.27 -> r1.28)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-protocol3.c.diff?r1=1.27&r2=1.28)