Fix table detection at beginning of buffer

See GH-373 and
#373 (comment)

Author: jrblevin

markdown-mode.el

@@ -8819,7 +8819,8 @@ This function assumes point is on a table."
     (while (and (not (bobp))
                 (markdown-table-at-point-p))
       (forward-line -1))
-    (unless (eobp)
+    (unless (or (eobp)
+                (markdown-table-at-point-p))
       (forward-line 1))
     (point)))