[magick-users] failed assertion `image->signature == MagickSignature'

[duc.sequere.aut.de.via.decede at imagemagick.org]

> failed assertion `image->signature == MagickSignature'

Methods that require an image, check to ensure the pointer being passed is
indeed an image by prescreening the pointer to ensure its signature member
is 0xabacadabUL.  If its not, some bogus pointer is being passed ensuring
disaster.