Clear core image if memory mapping was used for last load

Author: radarhere

Tests/test_tiff_crashes.py

@@ -52,3 +52,17 @@ def test_tiff_crashes(test_file: str) -> None:
         pytest.skip("test image not found")
     except OSError:
         pass
+
+
+def test_tiff_mmap() -> None:
+    try:
+        with Image.open("Tests/images/crash_mmap.tif") as im:
+            im.seek(1)
+            im.load()
+
+            im.seek(0)
+            im.load()
+    except FileNotFoundError:
+        if on_ci():
+            raise
+        pytest.skip("test image not found")

src/PIL/TiffImagePlugin.py

@@ -1217,9 +1217,10 @@ def seek(self, frame: int) -> None:
             return
         self._seek(frame)
         if self._im is not None and (
-            self.im.size != self._tile_size or self.im.mode != self.mode
+            self.im.size != self._tile_size
+            or self.im.mode != self.mode
+            or self.readonly
         ):
-            # The core image will no longer be used
             self._im = None
 
     def _seek(self, frame: int) -> None: