Fix a crash when cloning a volume with no backing store

Introduced by commit 15213d1e, not yet released.

src/storage/storage_backend.c

@@ -887,7 +887,7 @@ virStorageBackendCreateQemuImgCmd(virConnectPtr conn,
          * backing store, not really sure what use it serves though, and it
          * may cause issues with lvm. Untested essentially.
          */
-        if (inputvol &&
+        if (inputvol && inputvol->target.backingStore &&
             STRNEQ_NULLABLE(inputvol->target.backingStore->path,
                             vol->target.backingStore->path)) {
             virReportError(VIR_ERR_INTERNAL_ERROR, "%s",

tests/storagevolxml2argvdata/qcow2-convert-nobacking.argv

+qemu-img convert -f raw -O qcow2 -o encryption=on \
+/var/lib/libvirt/images/sparse.img /var/lib/libvirt/images/OtherDemo.img

tests/storagevolxml2argvtest.c

@@ -235,6 +235,9 @@ mymain(void)
     DO_TEST("pool-dir", "vol-qcow2-nobacking",
             "pool-dir", "vol-file",
             "qcow2-nobacking-convert-prealloc", flags, FMT_OPTIONS);
+    DO_TEST_FAIL("pool-dir", "vol-qcow2",
+                 "pool-dir", "vol-file",
+                 "qcow2-convert-nobacking", 0, FMT_OPTIONS);
     DO_TEST_FAIL("pool-dir", "vol-qcow2",
                  "pool-dir", "vol-file",
                  "qcow2-convert-prealloc", flags, FMT_OPTIONS);