box: fix out of bound write in error_payload_destroy()

If `strlen(name)` is 1, `value_size` is 1, and `extra` is 0, then 15 bytes
are allocated for `struct error_field` in error_payload_prepare(). However,
the size of this structure is 16 because of the padding for the alignment.
Thus TRASH() in error_payload_destroy() writes 1 byte beyond the structure.

Closes #9098

NO_DOC=bugfix