Security: Invalid write in FSS functions and ensure a reset used length on error.

Security: Invalid write in FSS functions and ensure a reset used length on error.

The command that triggers this is "fss_basic_write -oc hi there".

An invalid write is happening due to not always performing the array size increase operations.
Update all FSS write functions with more thorough checks.
This problem is being obfuscated by the lossy allocation.

When errors happen the destination->used needs to be consistently reset to the original value on return.

Use the lossy allocation as well for allocation shwere the increase by is a raw digit.
In these specific cases use state.step_small rather than state.step_large.

Rename used_start to destination_used for improved consistency.