fix(files_external): Check key exists before accessing it

Signed-off-by: Git'Fellow <12234510+solracsf@users.noreply.github.com>

apps/files_external/lib/Lib/Storage/SFTP.php

@@ -471,10 +471,14 @@ public function stat($path) {
 		try {
 			$stat = $this->getConnection()->stat($this->absPath($path));
 
-			$mtime = $stat ? (int)$stat['mtime'] : -1;
-			$size = $stat ? (int)$stat['size'] : 0;
-
-			return ['mtime' => $mtime, 'size' => $size, 'ctime' => -1];
+			$mtime = isset($stat['mtime']) ? (int)$stat['mtime'] : -1;
+			$size = isset($stat['size']) ? (int)$stat['size'] : 0;
+
+			return [
+				'mtime' => $mtime,
+				'size' => $size,
+				'ctime' => -1
+			];
 		} catch (\Exception $e) {
 			return false;
 		}