Check all reserved bytes in FLI header (#9183)

Author: hugovk

Tests/images/crash-5762152299364352.fli

@@ -49,7 +49,12 @@ class FliImageFile(ImageFile.ImageFile):
     def _open(self) -> None:
         # HEAD
         s = self.fp.read(128)
-        if not (_accept(s) and s[20:22] == b"\x00\x00"):
+        if not (
+            _accept(s)
+            and s[20:22] == b"\x00" * 2
+            and s[42:80] == b"\x00" * 38
+            and s[88:] == b"\x00" * 40
+        ):
             msg = "not an FLI/FLC file"
             raise SyntaxError(msg)