Fix the bug that inertias-scroll doesn't work because of the void variable 'follow-mode'

Sometimes, Emacs goes into the following condition:
  (symbol-plist 'follow-mode)
  => (event-symbol-element-mask (follow-mode 0) event-symbol-elements (follow-mode) modifier-cache ((0 . follow-mode)))

  (boundp 'follow-mode)
  => nil