Do not attempt to decode NULL SkData.

NewFromFD fails if mmap fails. In that case, it returns a NULL
SkData. SkMemoryStream handles NULL input by calling SkData::NewEmpty,
which is not threadsafe. If the SkMemoryStream were to get some
busted SkData, its call to read might fail. Sidestep this problem
by not creating the SkMemoryStream if the SkData is NULL, skipping
the call to SkData::NewEmpty.

BUG:11028218
Change-Id: Id70299bef1c85ffb5d17102fdb5ea071b0bee68a