don't show x-fade menu when right-clicking on frame handles

Fix crash with midi-regions (they have a frame-handle but no x-fade).
fixes 2nd part of #5992 (backtrace 20141021-B)

This is nicer in one way: When the cursor is "trim" the x-fade
context menu is no longer accessible.
And a bit worse: The x-fade context menu is only accessible on the
small fade-handles (boxes) and on the x-fade itself.