Still return quota value when free space is unknown

Fixed the quota storage wrapper to correctly return the quota value when
the free space is not known (which usually happens when the disk_free_space
function is disabled)

lib/private/files/storage/wrapper/quota.php

@@ -69,7 +69,14 @@ class Quota extends Wrapper {
 				return \OC\Files\SPACE_NOT_COMPUTED;
 			} else {
 				$free = $this->storage->free_space($path);
-				return min($free, (max($this->quota - $used, 0)));
+				$quotaFree = max($this->quota - $used, 0);
+				// if free space is known
+				if ($free >= 0) {
+					$free = min($free, $quotaFree);
+				} else {
+					$free = $quotaFree;
+				}
+				return $free;
 			}
 		}
 	}

tests/lib/files/storage/wrapper/quota.php

@@ -61,6 +61,24 @@ class Quota extends \Test\Files\Storage\Storage {
 		$this->assertEquals(6, $instance->free_space(''));
 	}
 
+	public function testFreeSpaceWithUnknownDiskSpace() {
+		$storage = $this->getMock(
+			'\OC\Files\Storage\Local',
+			array('free_space'),
+			array(array('datadir' => $this->tmpDir))
+		);
+		$storage->expects($this->any())
+			->method('free_space')
+			->will($this->returnValue(-2));
+		$storage->getScanner()->scan('');
+
+		$instance = new \OC\Files\Storage\Wrapper\Quota(array('storage' => $storage, 'quota' => 9));
+		$instance->getCache()->put(
+			'', array('size' => 3, 'unencrypted_size' => 0)
+		);
+		$this->assertEquals(6, $instance->free_space(''));
+	}
+
 	public function testFreeSpaceWithUsedSpaceAndEncryption() {
 		$instance = $this->getLimitedStorage(9);
 		$instance->getCache()->put(