Fix #135330: Incorrect pasted flipped armature keyframes

Caused by 6d832ee5b3

The bone names were flipped correctly, however, we were missing the
actual FCurve BezTriple flipping.
This was caused by passing the wrong FCurve into
`do_curve_mirror_flippping` -- this function is checking for existence
of the FCurve in the `KeyframeCopyBuffer`, so we gotta pass
`fcurve_in_copy_buffer` (instead of the new fcu).

Pull Request: https://projects.blender.org/blender/blender/pulls/135381

source/blender/editors/animation/keyframes_general.cc

@@ -1970,7 +1970,7 @@ static void paste_animedit_keys_fcurve(FCurve *fcu,
     BezTriple bezt_copy = *bezt;
 
     if (flip) {
-      do_curve_mirror_flippping(*fcu, bezt_copy);
+      do_curve_mirror_flippping(fcurve_in_copy_buffer, bezt_copy);
     }
 
     add_v2_v2(bezt_copy.vec[0], offset);