ffi-magic -- Fix #'magic:find-file-noselect unbound 'coding'

If you tried to open a file from a buffer where the coding system had
been set via file-local variable ("Local variables:" section or
cookie) it would result in a void variable error.  This change fixes
that.

	* lisp/ffi/ffi-magic.el (magic:find-file-noselect): let-bind
	'coding' inside the #'with-current-buffer instead of outside.

Signed-off-by: Steve Youngs <steve@sxemacs.org>