AttributeOutputBase::TextField() passes the address of its local
variable to WriteExpand(), and DocxAttributeOutput::WriteField_Impl()
stored this. When it was to be used, the variable already went out of
scope, resulting in a crash. Given that SwField is an abstract base
class, the easiest way is to just copy the field and manually delete it
when it's no longer needed.

Change-Id: I9d1fe2485277f1ac21a576d7ff0d05003f0ac8a1