backport fix crash:
- m_blockRange is a smart-range that highlights the block for the given codefolding region
- when the mouse is over the code folding border, m_blockRange is a valid smart range
- F5 reloads the document, which deletes the smart range
- now m_blockRange is a dangling pointer
- moving the mouse deletes this dangling pointer -> crash
CCBUG: 160527