pg_receivexlog - Exit on failure to parse

In streamutil.c:GetConnection(), upgrade failure to parse the
connection string to an exit(1) instead of simply returning NULL.
Most callers already immediately exited, but pg_receivexlog would
loop on this case, continually trying to re-parse the connection
string (which can't be changed after pg_receivexlog has started).

GetConnection() was already expected to exit(1) in some cases
(eg: failure to allocate memory or if unable to determine the
integer_datetimes flag), so this change shouldn't surprise anyone.

Began looking at this due to the Coverity scanner complaining that
we were leaking err_msg in this case- no longer an issue since we
just exit(1) immediately.