validate resource's integrity before using it

lib/private/image.php

@@ -283,9 +283,12 @@ class OC_Image {
 	}
 
 	/**
-	 * @return string Returns the raw image data.
+	 * @return null|string Returns the raw image data.
 	 */
 	function data() {
+		if (!$this->valid()) {
+			return null;
+		}
 		ob_start();
 		switch ($this->mimeType) {
 			case "image/png":

tests/lib/image.php

@@ -144,6 +144,11 @@ class Test_Image extends \Test\TestCase {
 		$this->assertEquals($expected, $img->data());
 	}
 
+	public function testDataNoResource() {
+		$img = new \OC_Image();
+		$this->assertNull($img->data());
+	}
+
 	/**
 	 * @depends testData
 	 */