Reject out-of-range numeric timezone specifications.

In commit 631dc390f49909a5c8ebd6002cfb2bcee5415a9d, we started to handle
simple numeric timezone offsets via the zic library instead of the old
CTimeZone/HasCTZSet kluge.  However, we overlooked the fact that the zic
code will reject UTC offsets exceeding a week (which seems a bit arbitrary,
but not because it's too tight ...).  This led to possibly setting
session_timezone to NULL, which results in crashes in most timezone-related
operations as of 9.4, and crashes in a small number of places even before
that.  So check for NULL return from pg_tzset_offset() and report an
appropriate error message.  Per bug #11014 from Duncan Gillis.

Back-patch to all supported branches, like the previous patch.
(Unfortunately, as of today that no longer includes 8.4.)