Do not unnecessarily update FLI __offset (#9184)

Author: hugovk

src/PIL/FliImagePlugin.py

@@ -48,6 +48,7 @@ class FliImageFile(ImageFile.ImageFile):
 
     def _open(self) -> None:
         # HEAD
+        assert self.fp is not None
         s = self.fp.read(128)
         if not (
             _accept(s)
@@ -82,8 +83,7 @@ def _open(self) -> None:
 
         if i16(s, 4) == 0xF100:
             # prefix chunk; ignore it
-            self.__offset = self.__offset + i32(s)
-            self.fp.seek(self.__offset)
+            self.fp.seek(self.__offset + i32(s))
             s = self.fp.read(16)
 
         if i16(s, 4) == 0xF1FA:
@@ -116,6 +116,7 @@ def _palette(self, palette: list[tuple[int, int, int]], shift: int) -> None:
         # load palette
 
         i = 0
+        assert self.fp is not None
         for e in range(i16(self.fp.read(2))):
             s = self.fp.read(2)
             i = i + s[0]