Ensure errors are reported when checking md5sums and sdcard is full.

When you do a backup from the recovery, md5 sums of the .img files are
created to allow confirmation of validity of backup. Backup failure is
checked by return code from nandroid-md5.sh, but the return code is just
that of md5sum.

If the SDCARD is (nearly) full, then zero length files can be created.
Zero length files have a valid md5sum: there is no error to report.

In these cases nandroid.md5 file will also be empty,
but there is no check on this in the nandroid-md5.sh script,
nor in the code used to check validity of the md5sums
(where md5sum -c nandroid.md5 is executed and returns no error).

I've not tested these on an android phone, but the script and the one line
shell command in nandroid.c have been tested on a Linux box where
sets of backup files (good and bad) had been copied.

Change-Id: Icc14fa32608832f7fc53c1235709fdee718ef0d5