HMP: Initialize err before using

When bdrv_snapshot_delete return fail, the errp will not be
assigned a valid value in error_propagate as errp didn't be
initialized in hmp_delvm, then error_reportf_err will use an
uninitialized value(call by hmp_delvm), and qemu crash.

Signed-off-by: zhangjixiang <jixiang_zhang@h3c.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

hmp.c

@@ -1321,7 +1321,7 @@ void hmp_savevm(Monitor *mon, const QDict *qdict)
 void hmp_delvm(Monitor *mon, const QDict *qdict)
 {
     BlockDriverState *bs;
-    Error *err;
+    Error *err = NULL;
     const char *name = qdict_get_str(qdict, "name");
 
     if (bdrv_all_delete_snapshot(name, &bs, &err) < 0) {