Fix error when trying to expand latex symbol at beginning of buffer

[dhanak]

With company-mode, the following error is thrown when typing (anything but a latex sequence) at the start of the buffer:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  =(92 nil)
  (if (= 92 (char-before)) (progn (- (point) 1)))
  (save-excursion (while (not (or (bobp) (= 92 (char-before)) (member (char-syntax (char-before)) '(32 60 62 92)))) (backward-char)) (if (= 92 (char-before)) (progn (- (point) 1))))
  julia--latexsub-start-symbol()
  julia-mode-latexsub-completion-at-point-around()
  completion--capf-wrapper(julia-mode-latexsub-completion-at-point-around optimist)
  company--capf-wrapper(julia-mode-latexsub-completion-at-point-around optimist)

This change fixes that.