Fix crash when right clicking AUI toolbar while left button is pressed.

Ignore the unrelated mouse events during the item dragging.

This avoids a crash due to m_actionItem being NULL when right clicking an item
while the left mouse button is kept pressed.

See #16327.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76746 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775