storage: Don't dereference driver object if virStorageSource is not initialized

virStorageFileReportBrokenChain uses data from the driver private data
pointer to print the user and group. This would lead to a crash in call
paths where we did not initialize the storage backend as recently added
in commit 24e47ee2 to qemuDomainDetermineDiskChain.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1522682

src/storage/storage_source.c

@@ -419,19 +419,33 @@ virStorageFileReportBrokenChain(int errcode,
                                 virStorageSourcePtr src,
                                 virStorageSourcePtr parent)
 {
-    unsigned int access_user = src->drv->uid;
-    unsigned int access_group = src->drv->gid;
-
-    if (src == parent) {
-        virReportSystemError(errcode,
-                             _("Cannot access storage file '%s' "
-                               "(as uid:%u, gid:%u)"),
-                             src->path, access_user, access_group);
+
+    if (src->drv) {
+        unsigned int access_user = src->drv->uid;
+        unsigned int access_group = src->drv->gid;
+
+        if (src == parent) {
+            virReportSystemError(errcode,
+                                 _("Cannot access storage file '%s' "
+                                   "(as uid:%u, gid:%u)"),
+                                 src->path, access_user, access_group);
+        } else {
+            virReportSystemError(errcode,
+                                 _("Cannot access backing file '%s' "
+                                   "of storage file '%s' (as uid:%u, gid:%u)"),
+                                 src->path, parent->path, access_user, access_group);
+        }
     } else {
-        virReportSystemError(errcode,
-                             _("Cannot access backing file '%s' "
-                               "of storage file '%s' (as uid:%u, gid:%u)"),
-                             src->path, parent->path, access_user, access_group);
+        if (src == parent) {
+            virReportSystemError(errcode,
+                                 _("Cannot access storage file '%s'"),
+                                 src->path);
+        } else {
+            virReportSystemError(errcode,
+                                 _("Cannot access backing file '%s' "
+                                   "of storage file '%s'"),
+                                 src->path, parent->path);
+        }
     }
 }