Fix wiping crash when block device is not present

Especially with /sd-ext, trying to wipe when the device is not
present causes a seg fault in the make_ext4fs function. Check to
make sure that the block device is present before trying to wipe.

Change-Id: I91973b5b832c9edbce81d61a271e84363c19e6c5
Merge remote-tracking branch 'omni/android-4.4' into android-4.4