[mutter] ui/frames: Drop the current grab info on button release

[Rui Matos <rtcm src gnome org>]

commit 1d2bead358284c43a388b8545d94aa5136fde6e1
Author: Rui Matos <tiagomatos gmail com>
Date:   Thu Jun 23 15:38:03 2016 +0200

    ui/frames: Drop the current grab info on button release
    
    This was added in commit d05b750b8dec3825d286e3ae30c41b41bd6aed98 and
    later removed inadvertently in commit
    d561b3b18f73a66bcd7947dce7ab1aab9e09965a .
    
    https://bugzilla.gnome.org/show_bug.cgi?id=767969

 src/ui/frames.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/ui/frames.c b/src/ui/frames.c
index 0e90567..0cb9e14 100644
--- a/src/ui/frames.c
+++ b/src/ui/frames.c
@@ -1114,6 +1114,7 @@ handle_button_release_event (MetaUIFrame *frame,
 {
   Display *display = GDK_DISPLAY_XDISPLAY (gdk_display_get_default ());
 
+  frame->frames->current_grab_op = META_GRAB_OP_NONE;
   meta_core_end_grab_op (display, event->time);
 
   /* We only handle the releases we handled the presses for (things