(encode_terminal_code): Free any previous memory blocks

(encode_terminal_code): Free any previous memory blocks
before calling xmalloc for encode_terminal_src or
encode_terminal_dst.
(maybe_fatal): Buffer argument deleted.  Don't free buffer here.
All callers changed.
(init_tty): Store termcap data and string buffers in new struct
tty members termcap_term_buffer and termcap_strings_buffer.
(delete_tty): Free them.
(syms_of_term): Initialize encode_terminal_src and
encode_terminal_dst.