Return a PyError instead of a fake fterror.

* Update Tests to IOError rather than OSError

Tests/oss-fuzz/test_fuzzers.py

@@ -57,6 +57,8 @@ def test_fuzz_fonts(path):
     with open(path, "rb") as f:
         try:
             fuzzers.fuzz_font(f.read())
-        except (Image.DecompressionBombError, Image.DecompressionBombWarning):
+        except (Image.DecompressionBombError,
+                Image.DecompressionBombWarning,
+                IOError):
             pass
         assert True

Tests/test_font_crash.py

@@ -17,6 +17,6 @@ class TestFontCrash:
 
     @skip_unless_feature("freetype2")
     def test_segfault(self):
-        with pytest.raises(OSError):
+        with pytest.raises(IOError):
             font = ImageFont.truetype("Tests/fonts/fuzz_font-5203009437302784")
             self._fuzz_font(font)

src/_imagingft.c

@@ -923,7 +923,7 @@ font_render(FontObject *self, PyObject *args) {
 
         // Null buffer, is dereferenced in FT_Bitmap_Convert
         if (!bitmap.buffer && bitmap.rows) {
-            return geterror(0x9D); // Bitmap missing
+            PyErr_SetString(PyExc_IOError, "Bitmap missing for glyph");
             goto glyph_error;
         }