Breaking sign leaves ghost sign until quit

Breaking a sign leaves a ghost sign if the chunk has been unloaded after placement, until quit and restart (SP) or log off and back on (server).

This bug does not always occur, but can mostly be reproduced as follows:

• place sign(s) against a block

• leave the loaded chunks area (i.e. tp far away), so they get unloaded

• return to the signs and break them, this should leave ghost signs (they are visible but not there, new signs can be placed, but old text shows)

• log off and back on (or quit and restart) and the ghosts are gone