Since commit d0ae244f, which
connects GimpChannel instances to their buffer's "changed" signal,
the XCF loading code that steals a channel's buffer when converting
it to a selection mask (which happens when loading an XCF with a
saved selection mask) is unsafe, since fails to perform the
necessary cleanup and setup of the buffer in the old and new
channel objects, respectively.

Perform the buffer transfer using the new
gimp_drawable_steal_buffer(), which does the same thing in a safe
manner.