Standard path not used as default

How to reproduce:
* Have a server with imagemagick installed on the system path (so seems not to need a special setting)
* generate image

=> error: Unable to generate the derived image

What makes this error really nasty (thus raising prio):
* Saving the image toolkit settings at admin/config/media/image-toolkit makes the error not reproducible any more.

This really shot me in the knee, because after every deployment on a new server, it errored out, but while debugging it went away.

Debugging showed it's just inconsistent use of variable get:

* in image_imagemagick_settings():

    '#default_value' => variable_get('imagemagick_convert', 'convert'),

* in _imagemagick_convert_exec():

    $convert_path = variable_get('imagemagick_convert', NULL);