issue0016: fix download error with big file

if the server do not support range/rest, when download a big file, the memory alloc onec time, that will case failed,
so write when cache failed

Change-Id: Id82956ed1810556917194bcedf79c089dd9a8dbf
Merge branch 'iceyer/issue0015-fix-download-error-with-big-file' into develop

Conflicts:
	src/pkg.linuxdeepin.com/transfer/ftp_client.go

Change-Id: I66da5415f090f2fdbcd306c86a3f7eaf28fe4964