prevent infinite loop

When `dante-eval-block` finds a comment at the beginning of the file, it previously entered an infinite loop.  See Issue jyp#182 for a full description of the problem and code blocks to illustrate the issue and fix.

dante.el

@@ -855,7 +855,7 @@ The command block is indicated by the >>> symbol."
                      ;; ensure that there is a newline at the end of buffer
                      (when (eq (point) (point-max)) (newline))
                      (point-marker))))
-    (while (looking-at "[ \t]*--") (forward-line -1))
+    (while (and (looking-at "[ \t]*--") (forward-line -1)))
     (forward-line)
     (lcr-spawn
       (lcr-call dante-async-load-current-buffer t nil)