Fix a resource leak: close fp before return (found by cppcheck)

[kathryndavies]

Minor fix for resource leak found by cppcheck.

[homm]

Please add the following test in Tests/test_image.py right after the test_internals:

    def test_internals(self):
        im = Image.new("L", (100, 100))
        im.readonly = 1
        im._copy()
        self.assertFalse(im.readonly)

        im.readonly = 1
        im.paste(0, (0, 0, 100, 100))
        self.assertFalse(im.readonly)

    def test_dump(self):
        im = Image.new("L", (10, 10))
        im._dump(self.tempfile("temp_L.ppm"))

        im = Image.new("RGB", (10, 10))
        im._dump(self.tempfile("temp_RGB.ppm"))

        im = Image.new("HSV", (10, 10))
        self.assertRaises(ValueError, im._dump, self.tempfile("temp_HSV.ppm"))

[homm]

Please, change line order: place fclose before ImagingError_ModeError.

[kathryndavies]

Thanks for the code review - I've made the changes.