afp: Fix afp backend crash when no username supplied
A missing NULL check in g_vfs_afp_command_put_pascal caused crashes (seg faults) when no username was given. Crashes happened irrespective of Password was given or not.
A missing NULL check in g_vfs_afp_command_put_pascal caused crashes (seg faults) when no username was given. Crashes happened irrespective of Password was given or not.