Fix infloop when user provides a wrong value for `org-highlight-latex-and-related'

Fix infloop when user provides a wrong value for `org-highlight-latex-and-related'

* lisp/org.el (org-do-latex-and-related): Fix infloop when user
  provides a wrong value for `org-highlight-latex-and-related'. In
  this case, `org-latex-and-related-regexp' is the empty string and
  generates an infloop since matching it doesn't move point.