Avoid a flyspell error if point is at beginning of buffer

Avoid a flyspell error if point is at beginning of buffer

* lisp/textmodes/flyspell.el (flyspell-generic-progmode-verify): Check
if point is at the beginning of the buffer.  This prevents an error
when e.g. 'flyspell-auto-correct-word' gets called with point at the
beginning of the buffer.

Bug#35967