inspection: Fix double-free when certain guest files are empty.

The following commit:

  commit 5a3da366268825b26b470cde35658b67c1d11cd4
  Author: Richard W.M. Jones <rjones@redhat.com>
  Date:   Thu Jan 24 17:07:38 2013 +0000

      inspect: Use CLEANUP_* macros in inspection code.

can cause a double-free along an error path when certain guest files
are empty where we expected those files to contain at least one line.

This causes virt-inspector to crash when run on these guests.

The following is a test case which demonstrates the crash.
`f20rawhidex64' is a Fedora guest, but with small adjustments to the
test you could use any Linux guest for this test.

  $ qemu-img create -f qcow2 -b f20rawhidex64 /tmp/test.qcow2
  Formatting '/tmp/test.qcow2', fmt=qcow2 size=21474836480 backing_file='f20rawhidex64' encryption=off cluster_size=65536 lazy_refcounts=off
  $ guestfish -i -a /tmp/test.qcow2 -- rm /etc/redhat-release : touch /etc/redhat-release
  $ virt-inspector /tmp/test.qcow2
  *** glibc detected *** virt-inspector: double free or corruption (fasttop): 0x00007f18bc9925a0 ***
  ======= Backtrace: =========
  /lib64/libc.so.6(+0x34ecc7ca8e)[0x7f18b8e64a8e]
  /lib64/libguestfs.so.0(+0x3f91898078)[0x7f18ba13c078]
  /lib64/libguestfs.so.0(+0x3f91899761)[0x7f18ba13d761]
  /lib64/libguestfs.so.0(+0x3f91896d12)[0x7f18ba13ad12]
  /lib64/libguestfs.so.0(+0x3f91894140)[0x7f18ba138140]
  /lib64/libguestfs.so.0(guestfs_inspect_os+0x35)[0x7f18ba0bcc35]
  virt-inspector(main+0x547)[0x7f18ba7c57d7]
  /lib64/libc.so.6(__libc_start_main+0xf5)[0x7f18b8e09a05]
  virt-inspector(+0x6665)[0x7f18ba7c7665]

This is a denial of service, but not likely to be exploitable.

(Found by Coverity)