Fix an oversight in my patch of a couple weeks ago that ensured a snapshot

is available during datatype input in Bind message processing.  I put the
PopActiveSnapshot() or equivalent just before PortalDefineQuery, which is
an unsafe spot for it (in 8.3 and later) because we are carrying a plancache
refcount that hasn't yet been assigned to the portal.  Any error thrown there
would result in leaking the refcount.  It's not exactly likely that
PopActiveSnapshot would throw an elog, perhaps, but it could happen.
Reorder the code and add another comment warning not to do that.