When committing a transformation to a floating-selection layer, if
the selection mask is non-empty (probably a state we don't want to
allow, but one that can currently be achieved), and the tool-GUI
dialog is detached, gimp_transform_grid_tool_response() is called
*while* applying the transformation, leading, ultimately, to a
segfault.
Fix this by returning early when this happens.
(cherry picked from commit 11efe64736)
84fdd1b601