Do not allow folder creation with quota of 0

Fixes #4577

Users with a quota of 0 are a special case. Since they can't (ever)
create files on their own storage. Therefor it makes no real that they
can create folders (and possible share those etc).

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

lib/private/Files/Storage/Wrapper/Quota.php

@@ -200,4 +200,12 @@ class Quota extends Wrapper {
 			return false;
 		}
 	}
+
+	public function mkdir($path) {
+		if ($this->quota === 0.0) {
+			return false;
+		}
+
+		return parent::mkdir($path);
+	}
 }

tests/lib/Files/Storage/Wrapper/QuotaTest.php

@@ -208,4 +208,9 @@ class QuotaTest extends \Test\Files\Storage\Storage {
 		$this->assertTrue($this->instance->instanceOfStorage('\OC\Files\Storage\Wrapper\Wrapper'));
 		$this->assertTrue($this->instance->instanceOfStorage('\OC\Files\Storage\Wrapper\Quota'));
 	}
+
+	public function testNoMkdirQuotaZero() {
+		$instance = $this->getLimitedStorage(0.0);
+		$this->assertFalse($instance->mkdir('foobar'));
+	}
 }