domain_conf: fix NULL dereference on error in virDomainObjCopyPersistentDef

domain_conf: fix NULL dereference on error in virDomainObjCopyPersistentDef

The issue was introduced together with the function itself by commit
<da1eba6bc8f58bfce34136710d1979a3a44adb17>.  Calling
`virDomainObjGetPersistentDef` may return NULL which is later passed
to `virDomainDefFormat` where the `def` attribute is marked as NONNULL
and later in `virDomainDefFormatInternalSetRootName` it is actually
defererenced without any other check.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>