Fix T74516: Armature Crash on Select Similar Group

Select Similar Group and Select Similar Shape had this issue since they
were added. Basically it assumes there is pose data which in some cases
it does not.

source/blender/editors/armature/armature_select.c

@@ -1461,6 +1461,13 @@ static void select_similar_data_pchan(bContext *C, const size_t bytes_size, cons
   EditBone *ebone_act = CTX_data_active_bone(C);
 
   const bPoseChannel *pchan_active = BKE_pose_channel_find_name(obedit->pose, ebone_act->name);
+
+  /* This will mostly happen for corner cases where the user tried to access this
+   * before having any valid pose data for the armature. */
+  if (pchan_active == NULL) {
+    return;
+  }
+
   const char *data_active = (const char *)POINTER_OFFSET(pchan_active, offset);
   for (EditBone *ebone = arm->edbo->first; ebone; ebone = ebone->next) {
     if (EBONE_SELECTABLE(arm, ebone)) {