various fixes

[Justus Winter]

Hi :)

[PATCH 1/3] ext2fs: remove inappropriate assertion

This is an rfc.  I had a root fs die because this assertion failed,
and from what I have seen it is not appropriate to assert (block)
here.  Then again, this pager business is all new to me and I'm not
really sure if it is okay to skip blocks like this patch does or if we
should instead write a block of zeros here.

[PATCH 2/3] isofs: also copy the terminating zero in
[PATCH 3/3] libdiskfs: fix string termination

To everyone who wants to help out with the Hurd development: Looking
through the static analyzer reports is a great way to get started if
you know c but are otherwise new to the Hurd.  There are still some
low-hanging reports left, like the one this patch is addressing:

http://darnassus.sceen.net/~teythoon/qa/hurd/scan-build/2014-02-21/report-Y64Nfa.html#EndPath

Justus