Commits 51082301, 16d7b39, and 521cc447 introduced support for
'virsh snapshot-list --from' when talking to a server older than
0.9.5, but broke support for plain 'virsh snapshot-list' for the
same old server in the process. Because the code is not properly
gated, we end up with a SIGSEGV during a strcmp with a NULL argument.
* tools/virsh.c (cmdSnapshotList): Don't waste time on fallbacks
when --from is not present.
(cherry picked from commit d9f606659829e666803d95d05aee9ca1ecced955)