Hi Eli, Yuan and others,

mark-defun errors when node is missing in the call to goto-char.

Repro:

emacs -Q
find-file xdisp.c
M-g M-g 450 RET
M-x mark-defun

Observe that emacs is erroring.

This patch fixes this bug.  However, I notice that
treesit-beginning-of-defun returns t after it is done.  Is there a
reason for end-of-defun not to return the same?

Theo