fix #15973

Rework of stream_seek handling; there where basically two bugs: 1. seeking to the end of the current file would fail (with SEEK_SET); and 2. if seeking to an undefined position (outside 0,unencryptedSize) then newPosition was not defined. I used the opportunity to simplify the code.

lib/private/files/stream/encryption.php

@@ -356,24 +356,22 @@ class Encryption extends Wrapper {
 
 		switch ($whence) {
 			case SEEK_SET:
-				if ($offset < $this->unencryptedSize && $offset >= 0) {
-					$newPosition = $offset;
-				}
+				$newPosition = $offset;
 				break;
 			case SEEK_CUR:
-				if ($offset >= 0) {
-					$newPosition = $offset + $this->position;
-				}
+				$newPosition = $this->position + $offset;
 				break;
 			case SEEK_END:
-				if ($this->unencryptedSize + $offset >= 0) {
-					$newPosition = $this->unencryptedSize + $offset;
-				}
+				$newPosition = $this->unencryptedSize + $offset;
 				break;
 			default:
 				return $return;
 		}
 
+		if ($newPosition > $this->unencryptedSize || $newPosition < 0) {
+			return $return;
+		}
+
 		$newFilePosition = floor($newPosition / $this->unencryptedBlockSize)
 			* $this->util->getBlockSize() + $this->headerSize;