fix savevm command without id or tag

savevm without id or tag segfaults in:

(gdb) bt
#0  0x00007f600a83bf8a in __strcmp_sse42 () from /lib64/libc.so.6
#1  0x00000000004745b6 in bdrv_snapshot_find (bs=<value optimized out>,
    sn_info=0x7fff996be280, name=0x0) at savevm.c:1631
#2  0x0000000000475c80 in del_existing_snapshots (name=<value optimized out>,
    mon=<value optimized out>) at savevm.c:1654
#3  do_savevm (name=<value optimized out>, mon=<value optimized out>)
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

savevm.c

@@ -1695,7 +1695,7 @@ void do_savevm(Monitor *mon, const QDict *qdict)
     sn->vm_clock_nsec = qemu_get_clock(vm_clock);
 
     /* Delete old snapshots of the same name */
-    if (del_existing_snapshots(mon, name) < 0) {
+    if (name && del_existing_snapshots(mon, name) < 0) {
         goto the_end;
     }