pinning freespace to a non-freespaced label is broken

```
!rb as2
data_bank = $808000
org data_bank
db $00

segment pin=data_bank
db $00

[12:54] jimblerbot999:
Errors occurred while assembling.
<input>:7: error: (Esnes_address_doesnt_map_to_rom): SNES address 000000 doesn't map to ROM.
    in block: [db $00]
```