installer: Fix data size

Previously the installer backend didn't handl eincorrect data_size
values correctly. Now the data_size value is checked and adjusted if
necessary.

Change-Id: I724bae27a4078c10885cfb215476d3a8b116e25f
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1022380
Signed-off-by: Fabian Deutsch <fabiand@redhat.com>