don't interpret single "-" as a number

* lisp/ob-core.el (org-babel-number-p): Don't interpret single "-" as a
  number.

lisp/ob-core.el

@@ -2521,6 +2521,7 @@ appropriate."
 (defun org-babel-number-p (string)
   "If STRING represents a number return its value."
   (if (and (string-match "^-?[0-9]*\\.?[0-9]*$" string)
+	   (string-match "[0-9]+" string)
            (= (length (substring string (match-beginning 0)
 				 (match-end 0)))
 	      (length string)))