textmodes/flyspell.el: Don't check pre-word if buffer was switched.

textmodes/flyspell.el: Don't check pre-word if buffer was switched.

If command changed the buffer, the decision may be made based on the
current buffer even though it should based on the previous one.  This
may lead to false positives and more importantly to errors since
`flyspell-pre-point' is buffer local so it may have unsanitised value
(such as nil) in previous buffer.

To be honest, I'm not sure how this can happen since
`flyspell-pre-point' is set in previous buffer, but nonetheless, I've
been encountering the error for quite some time and finally decided to
fix it.  Interestingly, line making `flyspell-pre-point'
a buffer-local variable has a very revealing "Why?? --Stef" comment.

To avoid the problem, change flyspell-check-pre-word-p so that it does
not allow checking of pre-word if command changed buffer
(ie. `flyspell-pre-buffer' is not current buffer).