Fix #118225 possible crash double clicking on certain anim channels

The intend in the report was to rename a linked object in the NLA
channels. The operator to rename actually does everything right, it
detects that the object is linked and passes through to other operators.

The crash then happens in the attempt to select keyframes of channels
(also doubleclick n the keymap) -- in `select_anim_channel_keys`.

The problem is that `bAnimListElem` `key_data` cannot be expected to be
an `FCurve` in all cases. Code does though, it always casts, but this is
unreliable, basically this would fail for all of the "summary" channels
or any channel type (e.g. mask layers etc.), either it is NULL or
garbage data.

So to resolve, we just check the `bAnimListElem` type -- if it is not
ALE_FCURVE we can early out (preventing the crash).

NOTE: this changes behavior of double clicking on a summary channel that
cannot be renamed slightly in that it will not deselect its keys anymore
(it was not selecting any keys anyways, this is actually more in line of
what would happen if you just select another channel -- this also leaves
key selection alone -- so this is actually an improvement as well imho)

Pull Request: https://projects.blender.org/blender/blender/pulls/118251

source/blender/editors/animation/anim_channels_edit.cc

@@ -4029,6 +4029,12 @@ static bool select_anim_channel_keys(bAnimContext *ac, int channel_index, bool e
     return false;
   }
 
+  /* Only FCuves can have their keys selected. */
+  if (ale->datatype != ALE_FCURVE) {
+    ANIM_animdata_freelist(&anim_data);
+    return false;
+  }
+
   fcu = (FCurve *)ale->key_data;
   success = (fcu != nullptr);