Don't check the validity of newly contructed data buffers

A bunch of places were checking that DataBufferHeap::GetBytes returns a
non-null pointer right after constructing it. The only time when
GetBytes returns a null pointer is if it is empty (and I'm not sure that
even this is a good idea), but that is clearly not the case here, as the
buffer was constructed with a non-zero size just a couple of lines back.

llvm-svn: 364754