Update old-point in go-eldoc--goto-beginning-of-funcall between iterations

The current behaviour only works because of an unintended side-effect
in go-goto-opening-parenthesis, where it will jump to the beginning of
the buffer if point isn't in a balanced pair of parentheses. Future
versions of go-goto-opening-parenthesis will not move point at all.

With the change to go-goto-opening-parenthesis, the old implementation
of go-eldoc--goto-beginning-of-funcall could end up in an infinite
loop.