xenconfig: fix handling of NULL disk source

It is possible to crash libvirtd when converting xl native config to
domXML when the xl config contains an empty disk source, e.g. an empty
CDROM. Fix by checking that the disk source is non-NULL before parsing it.
Signed-off-by: NWim ten Have <wim.ten.have@oracle.com>

src/xenconfig/xen_xl.c

@@ -316,6 +316,10 @@ xenParseXLDiskSrc(virDomainDiskDefPtr disk, char *srcstr)
     char *tmpstr = NULL;
     int ret = -1;
 
+    /* A NULL source is valid, e.g. an empty CDROM */
+    if (srcstr == NULL)
+        return 0;
+
     if (STRPREFIX(srcstr, "rbd:")) {
         if (!(tmpstr = virStringReplace(srcstr, "\\\\", "\\")))
             goto cleanup;

tests/xlconfigdata/test-disk-positional-parms-partial.cfg

@@ -22,4 +22,4 @@ parallel = "none"
 serial = "none"
 builder = "hvm"
 boot = "d"
-disk = [ "/dev/HostVG/XenGuest2,,hda,,backendtype=phy", "/var/lib/libvirt/images/XenGuest2-home,,hdb,,", "/root/boot.iso,,hdc,,devtype=cdrom" ]
+disk = [ "/dev/HostVG/XenGuest2,,hda,,backendtype=phy", "/var/lib/libvirt/images/XenGuest2-home,,hdb,,", "/root/boot.iso,,hdc,,devtype=cdrom" , "format=raw,vdev=hdd,access=ro,devtype=cdrom" ]

tests/xlconfigdata/test-disk-positional-parms-partial.xml

@@ -39,6 +39,12 @@
       <readonly/>
       <address type='drive' controller='0' bus='1' target='0' unit='0'/>
     </disk>
+    <disk type='file' device='cdrom'>
+      <driver name='qemu' type='raw'/>
+      <target dev='hdd' bus='ide'/>
+      <readonly/>
+      <address type='drive' controller='0' bus='1' target='0' unit='1'/>
+    </disk>
     <controller type='ide' index='0'/>
     <interface type='bridge'>
       <mac address='00:16:3e:66:92:9c'/>