selection: ignore drag-and-drops with unsupported mime-types

Specifically, make sure we do *not* call wl_data_offer_receive() with
a NULL mime-type, as this causes libwayland to error out, which in
turn causes foot to exit.

Closes #1092

CHANGELOG.md

@@ -74,10 +74,15 @@
 
 * Graphical corruption when viewport is at the top of the scrollback,
   and the output is scrolling.
-* Improved text reflow of logical lines with trailing empty cells ([#1055][1055])
+* Improved text reflow of logical lines with trailing empty cells
+  ([#1055][1055])
 * IME focus is now tracked independently from keyboard focus.
+* Workaround for buggy compositors (e.g. some versions of GNOME)
+  allowing drag-and-drops even though foot has reported it does not
+  support the offered mime-types ([#1092][1092]).
 
 [1055]: https://codeberg.org/dnkl/foot/issues/1055
+[1092]: https://codeberg.org/dnkl/foot/issues/1092
 
 
 ### Security

selection.c

@@ -2392,6 +2392,12 @@ drop(void *data, struct wl_data_device *wl_data_device)
 
     struct wl_clipboard *clipboard = &seat->clipboard;
 
+    if (clipboard->mime_type == DATA_OFFER_MIME_UNSET) {
+        LOG_WARN("compositor called data_device::drop() "
+                 "even though we rejected the drag-and-drop");
+        return;
+    }
+
     struct dnd_context *ctx = xmalloc(sizeof(*ctx));
     *ctx = (struct dnd_context){
         .term = term,