mu4e: handle marks outside headers-view

Ensure we the marks get counted correctly outside the headers view. This
avoids mu4e trying to refresh the headers buffer when we're reading a
message.

mu4e/mu4e-mark.el

@@ -438,8 +438,8 @@ If NO-CONFIRMATION is non-nil, don't ask user for confirmation."
 
 (defun mu4e-mark-marks-num ()
   "Return the number of marks in the current buffer."
-  (if mu4e~mark-map (hash-table-count mu4e~mark-map) 0))
-
+  (mu4e~mark-in-context
+    (if mu4e~mark-map (hash-table-count mu4e~mark-map) 0)))
 
 (defun mu4e-mark-handle-when-leaving ()
   "If there are any marks in the current buffer, handle those