qemuDomainDiskByName() can return a NULL pointer on failure.
But this returned value in qemuSnapshotDeleteValidate is not checked.It will make libvirtd crash.

Signed-off-by: kaihuan <jungleman759@gmail.com>
---
 src/qemu/qemu_snapshot.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_snapshot.c b/src/qemu/qemu_snapshot.c
index 18b2e478f6..bcbd913073 100644
--- a/src/qemu/qemu_snapshot.c
+++ b/src/qemu/qemu_snapshot.c
@@ -4242,8 +4242,19 @@ qemuSnapshotDeleteValidate(virDomainObj *vm,
             virDomainDiskDef *vmdisk = NULL;
             virDomainDiskDef *disk = NULL;
 
-            vmdisk = qemuDomainDiskByName(vm->def, snapDisk->name);
-            disk = qemuDomainDiskByName(snapdef->parent.dom, snapDisk->name);
+            if (!(vmdisk = qemuDomainDiskByName(vm->def, snapDisk->name))) {
+                virReportError(VIR_ERR_OPERATION_FAILED,
+                            _("disk '%1$s' referenced by snapshot '%2$s' not found in the current definition"),
+                            snapDisk->name, snap->def->name);
+                return -1;
+            }
+
+            if (!(disk = qemuDomainDiskByName(snapdef->parent.dom, snapDisk->name))) {
+                virReportError(VIR_ERR_OPERATION_FAILED,
+                            _("disk '%1$s' referenced by snapshot '%2$s' not found in the VM definition of the deleted snapshot"),
+                            snapDisk->name, snap->def->name);
+                return -1;
+            }
 
             if (!virStorageSourceIsSameLocation(vmdisk->src, disk->src)) {
                 virReportError(VIR_ERR_OPERATION_UNSUPPORTED,
-- 
2.33.1.windows.1