Fix tzdata loading.

Android uses one large file of tzdata, so the "read up to the maximum
possible length of a valid tzfile" code in upstream localtime.c is
broken: there is always data after the current tzfile (even the
last tzfile is followed by the zone.tab data). This patch passes the
exact length through to the read(2) call so we don't over-read, rather
than have to rewrite upstream code that measures back from the "end" of
the tzfile.

The old code failed the existing time.strftime_null_tm_zone test after
updating to tzdata2016g.

Bug: http://b/31848040
Test: time.strftime_null_tm_zone
Change-Id: Iee059b5a8c051bd4952cfd80f02b00d83e489d5e