Fix Pull Mirror out-of-sync bugs (#24732) (#24733)

Backport #24732 

Bug-fix only

Author: wxiaoguang

templates/repo/header.tmpl

@@ -39,7 +39,7 @@
 						{{end}}
 					</div>
 				</div>
-				{{if $.IsPullMirror}}
+				{{if and $.IsPullMirror $.Mirror}}
 					{{$address := MirrorRemoteAddress $.Context . $.Mirror.GetRemoteName false}}
 					<div class="fork-flag">{{$.locale.Tr "repo.mirror_from"}} <a target="_blank" rel="noopener noreferrer" href="{{$address.Address}}">{{$address.Address}}</a></div>
 				{{end}}

templates/repo/settings/options.tmpl

@@ -88,7 +88,7 @@
 						</tr>
 					</thead>
 					{{end}}
-					{{if .Repository.IsMirror}}
+					{{if and .Repository.IsMirror .Mirror}}
 					<tbody>
 						<tr>
 							<td>{{(MirrorRemoteAddress $.Context .Repository .Mirror.GetRemoteName false).Address}}</td>