fix SeekableHttpStream::stream_stat errors on failed open

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

[skip ci]

Author: icewind1991

lib/private/Files/Stream/SeekableHttpStream.php

@@ -219,7 +219,9 @@ public function stream_tell() {
 	public function stream_stat() {
 		if ($this->getCurrent()) {
 			$stat = fstat($this->getCurrent());
-			$stat['size'] = $this->totalSize;
+			if ($stat) {
+				$stat['size'] = $this->totalSize;
+			}
 			return $stat;
 		} else {
 			return false;