Fix P -> PA conversion

Tests/test_image_convert.py

@@ -222,6 +222,20 @@ def test_p_la():
     assert_image_similar(alpha, comparable, 5)
 
 
+def test_p2pa_alpha():
+    with Image.open("Tests/images/tiny.png") as im:
+        assert im.mode == "P"
+
+        im_pa = im.convert("PA")
+    assert im_pa.mode == "PA"
+
+    im_a = im_pa.getchannel("A")
+    for x in range(4):
+        alpha = 255 if x > 1 else 0
+        for y in range(4):
+            assert im_a.getpixel((x, y)) == alpha
+
+
 def test_matrix_illegal_conversion():
     # Arrange
     im = hopper("CMYK")

src/libImaging/Convert.c

@@ -1031,7 +1031,7 @@ p2pa(UINT8 *out, const UINT8 *in, int xsize, ImagingPalette palette) {
     int x;
     int rgb = strcmp(palette->mode, "RGB");
     for (x = 0; x < xsize; x++, in++) {
-        const UINT8 *rgba = &palette->palette[in[0]];
+        const UINT8 *rgba = &palette->palette[in[0] * 4];
         *out++ = in[0];
         *out++ = in[0];
         *out++ = in[0];