fix FLI/FLC decoder for files with a prefix chunk

Tests/test_file_fli.py

@@ -4,17 +4,20 @@
 
 import pytest
 
-from PIL import FliImagePlugin, Image
+from PIL import FliImagePlugin, Image, ImageFile
 
 from .helper import assert_image_equal, assert_image_equal_tofile, is_pypy
 
 # created as an export of a palette image from Gimp2.6
 # save as...-> hopper.fli, default options.
 static_test_file = "Tests/images/hopper.fli"
 
-# From https://samples.libav.org/fli-flc/
+# From https://samples.ffmpeg.org/fli-flc/
 animated_test_file = "Tests/images/a.fli"
 
+# From https://samples.ffmpeg.org/fli-flc/
+animated_flc_with_prefixchunk_test_file = "Tests/images/2422.flc"
+
 
 def test_sanity() -> None:
     with Image.open(static_test_file) as im:
@@ -31,6 +34,19 @@ def test_sanity() -> None:
         assert im.info["duration"] == 71
         assert im.is_animated
 
+    ImageFile.LOAD_TRUNCATED_IMAGES = True
+    with Image.open(animated_flc_with_prefixchunk_test_file) as im:
+        assert im.mode == "P"
+        assert im.size == (320, 200)
+        assert im.format == "FLI"
+        assert im.info["duration"] == 171
+        assert im.is_animated
+        palette = im.getpalette()
+        assert (palette[3], palette[4], palette[5]) == (255, 255, 255)
+        assert (palette[381], palette[382], palette[383]) == (204, 204, 12)
+        assert (palette[765], palette[766], palette[767]) == (252, 0, 0)
+    ImageFile.LOAD_TRUNCATED_IMAGES = False
+
 
 @pytest.mark.skipif(is_pypy(), reason="Requires CPython")
 def test_unclosed_file() -> None:

src/PIL/FliImagePlugin.py

@@ -77,6 +77,7 @@ def _open(self):
         if i16(s, 4) == 0xF100:
             # prefix chunk; ignore it
             self.__offset = self.__offset + i32(s)
+            self.fp.seek(self.__offset)
             s = self.fp.read(16)
 
         if i16(s, 4) == 0xF1FA: