macho: fix bug when freeing Decl

Take into account that an already freed Decl will no longer be
available as `decl.link.macho` causing a potential "inactive union field"
panic.
d0edd37f69Jakub Konka committed on 7/21/2021, 9:38:20 PMparente05b1e0
1 file changedLine totals unavailable