fix: don't error when moving a non-existing mount

Signed-off-by: Robin Appelman <robin@icewind.nl>

Author: icewind1991

lib/private/Files/Mount/Manager.php

@@ -59,8 +59,10 @@ public function removeMount(string $mountPoint): void {
 	}
 
 	public function moveMount(string $mountPoint, string $target): void {
-		$this->mounts[$target] = $this->mounts[$mountPoint];
-		unset($this->mounts[$mountPoint]);
+		if (isset($this->mounts[$mountPoint])) {
+			$this->mounts[$target] = $this->mounts[$mountPoint];
+			unset($this->mounts[$mountPoint]);
+		}
 		$this->pathCache->clear();
 		$this->inPathCache->clear();
 		$this->areMountsSorted = false;