When using fromTimeStamp with utc=true, make sure to initialise the timezone to UTC

[derickr]

Without this change, the following code:

c_dt = HPHP::Unit::lookupClass(HPHP::s_DateTime.get());
assert(c_dt);
HPHP::ObjectData* obj = HPHP::ObjectData::newInstance(c_dt);

DateTimeData* data = Native::data<DateTimeData>(obj);
data->m_dt = makeSmartPtr<DateTime>(0, false);
data->m_dt->fromTimeStamp(milliseconds / 1000, true);

Would cause issues when var_dumping the returned object. var_dump would call
debugInfo on the object, which in turns tries to use zone_type_to_string, which
has an assertion if it can't find the type. With the above example, the zone
type would be false, and we'd get this assertion:

Program received signal SIGABRT, Aborted.
0x00007fffeeae5107 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56  ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  0x00007fffeeae5107 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1  0x00007fffeeae64e8 in __GI_abort () at abort.c:89
#2  0x00000000027ef459 in HPHP::assert_fail (e=e@entry=0x2eefd44 "!\"Bad zone type\"", 
    file=file@entry=0x2f7b278 "/home/derick/dev/facebook-hhvm/hphp/runtime/ext/datetime/ext_datetime.cpp", line=line@entry=59, 
    func=func@entry=0x4081380 <HPHP::zone_type_to_string(int, HPHP::SmartPtr<HPHP::DateTime>)::__PRETTY_FUNCTION__> "HPHP::String HPHP::zone_type_to_string(int, HPHP::SmartPtr<HPHP::DateTime>)", msg="") at /home/derick/dev/facebook-hhvm/hphp/util/assertions.cpp:80
#3  0x00000000023c18cb in HPHP::zone_type_to_string (zoneType=<optimized out>, dt=...) at /home/derick/dev/facebook-hhvm/hphp/runtime/ext/datetime/ext_datetime.cpp:59
#4  0x00000000023c6222 in HPHP::DateTimeData::getDebugInfo (this=0x7fffe5c1d438) at /home/derick/dev/facebook-hhvm/hphp/runtime/ext/datetime/ext_datetime.cpp:307
#5  0x00000000015250d7 in HPHP::ObjectData::toArray (this=0x7fffe5c1d440, pubOnly=pubOnly@entry=false) at /home/derick/dev/facebook-hhvm/hphp/runtime/base/object-data.cpp:442
#6  0x00000000014861e2 in HPHP::Object::toArray (this=this@entry=0x7fffffffca40) at /home/derick/dev/facebook-hhvm/hphp/runtime/base/type-object.cpp:49

[facebook-github-bot]

This pull request has been imported into Phabricator, and discussion and review of the diff will take place at https://reviews.facebook.net/D40803