fix reading from empty buffer when loading .gbr

Author: ElinksFr

Tests/test_file_gbr.py

@@ -15,3 +15,10 @@ def test_gbr_file():
     with Image.open("Tests/images/gbr.gbr") as im:
         with Image.open("Tests/images/gbr.png") as target:
             assert_image_equal(target, im)
+
+
+def test_multiples_operation():
+    with Image.open("Tests/images/gbr.gbr") as im:
+        rect = (0, 0, 10, 10)
+        im.crop(rect)
+        im.crop(rect)

src/PIL/GbrImagePlugin.py

@@ -84,8 +84,9 @@ def _open(self):
         self._data_size = width * height * color_depth
 
     def load(self):
-        self.im = Image.core.new(self.mode, self.size)
-        self.frombytes(self.fp.read(self._data_size))
+        if not self.im:
+            self.im = Image.core.new(self.mode, self.size)
+            self.frombytes(self.fp.read(self._data_size))
 
 
 #