imagemagick: security bump to version 6.9.4-1

Fixes:
Fix GetNextToken() off by one error.
Check for buffer overflow in magick/draw.c/DrawStrokePolygon().
Remove support for internal ephemeral coder.

These are all related to the recent ImageTragick bundle that were
partially fixed in 6.9.3-10 as well.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>