app: zero-out transparent pixels when converting to indexed

When converting an image to indexed mode, zero-out transparent
pixels instead of leaving junk in their indices, which might well
be out of range of the palette.

(cherry picked from commit 09870d4b15)

app/core/gimpimage-convert-indexed.c

@@ -714,6 +714,8 @@ remap_indexed_layer (GimpLayer    *layer,
             {
               if (data[ALPHA_I])
                 data[INDEXED] = remap_table[data[INDEXED]];
+              else
+                data[INDEXED] = 0;
 
               data += bpp;
             }