Clear C image when MPO frame image size changes

Tests/test_file_mpo.py

@@ -104,16 +104,16 @@ def test_exif(test_file: str) -> None:
 
 
 def test_frame_size() -> None:
-    # This image has been hexedited to contain a different size
-    # in the SOF marker of the second frame
-    with Image.open("Tests/images/sugarshack_frame_size.mpo") as im:
-        assert im.size == (640, 480)
+    with Image.open("Tests/images/frame_size.mpo") as im:
+        assert im.size == (56, 70)
+        im.load()
 
         im.seek(1)
-        assert im.size == (680, 480)
+        assert im.size == (349, 434)
+        im.load()
 
         im.seek(0)
-        assert im.size == (640, 480)
+        assert im.size == (56, 70)
 
 
 def test_ignore_frame_size() -> None:

src/PIL/JpegImagePlugin.py

@@ -193,6 +193,8 @@ def SOF(self: JpegImageFile, marker: int) -> None:
     n = i16(self.fp.read(2)) - 2
     s = ImageFile._safe_read(self.fp, n)
     self._size = i16(s, 3), i16(s, 1)
+    if self._im is not None and self.size != self.im.size:
+        self._im = None
 
     self.bits = s[0]
     if self.bits != 8: