The crash was coincidentally fixed in this upstream commit:

commit 090404acfeee1bdaec65d8c74137508467f979e6
Author: Daniel P. Berrange <berrange>
Date:   Thu Oct 21 13:14:33 2010 +0100

which was included in libvirt-0.8.5.

The question still remains about error reporting. For some reason, libvirt previously attempted to log an error when the IP address of a static host map was omitted, but then still continued (or rather, tried to continue, but crashed); now it completely ignores the entry with no IP address, and it's lost from the config.

It seems more useful to log an error and stop the parse when this happens, so that the user knows they've done something wrong.

I've also changed the parser to log an error and stop parsing if a static hosts entry is missing the IP address, or if it has neither a mac or name attribute (one or the other is required. That patch was posted upstream:

https://www.redhat.com/archives/libvir-list/2011-January/msg00114.html

and committed:

commit 6741ca36bd260691362c3198b59554855e879f8c
Author: Laine Stump <laine>
Date:   Mon Jan 3 16:47:17 2011 -0500

    Improve error reporting when parsing dhcp info for virtual networks
    
    This is partially in response to
    
      https://bugzilla.redhat.com/show_bug.cgi?id=653300
    
    The crash in that report was coincidentally fixed when we switched
    from using inet_pton() to using virSocketParseAddr(), but the absence
    of an ip address in a dhcp static host definition was still silently
    ignored (and that entry discarded from the saved XML). This patch
    turns that into a logged failure; likewise if the entry has neither a
    mac address nor a name attribute (the entry is useless without at
    least one of those, plus an ip address).
    
    Since the network name is now pulled into this function in order for
    those error logs to be more informative, the other error messages in
    the function have also been changed to take advantage.

This will also be in the upcoming rebase of libvirt based on the next libvirt release, so I'm moving it to POST.

Verified it on RHEL6 server with the following components:
libvirt-0.8.7-2.el6
qemu-kvm-0.12.1.2-2.129.el6
kernel-2.6.32-94.el6

The steps are as follows:
1 prepare the bellow network xml:
# cat invalid_network.xml
<network>
  <name>s3</name>
  <forward mode='nat'/>
  <bridge name='s3' stp='on' delay='0' />
  <ip address='192.168.133.1' netmask='255.255.255.0'>
      <dhcp>
            <range start='192.168.133.2' end='192.168.133.254' />
            <host mac='52:54:00:ef:88:9d' name='s3-host2' />
      </dhcp>
  </ip>
</network>

2 define network
# virsh define invalid_network.xml
error: Failed to define domain from invalid_network.xml
error: internal error incorrect root element

After step 2, check libvirtd status
# service libvirtd status
libvirtd (pid  18257) is running...

*** Bug 688087 has been marked as a duplicate of this bug. ***

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHBA-2011-0596.html