resize2fs: check for bogus new_size

resize2fs: check for bogus new_size

If we don't check for new_size == 0, bogus values send resize2fs into
a tailspin:

resize2fs 1.41.0 (10-Jul-2008)
Illegal block number passed to ext2fs_test_block_bitmap #1 for block bitmap for
/tmp/tmp.lntZtMFvz8/fake-disk
...the same message repeated zillion times...

Probably should see where that loop is, but at any rate we should
error-check parse_num_blocks.

Thanks to Petr Muller for reporting this.

Addresses-Red-Hat-Bugzilla: #465984

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>