_completion_loader: Use _longopt as the fallback.

bash_completion

@@ -1836,8 +1836,8 @@ _completion_loader()
     # Finally, try basename.
     . "$compdir/${1##*/}" &>/dev/null && return 124
 
-    # Fall back to something generic if we don't have anything else.
-    complete -o bashdefault -f "$1" && return 124
+    # Need to define *something*, otherwise there will be no completion at all.
+    complete -F _longopt "$1" && return 124
 } &&
 complete -D -F _completion_loader