Fix unlikely core dump in init_tty, and simplify terminfo case.
* term.c (init_tty) [TERMINFO]: Fix check for buffer overrun.
The old version incorrectly dumped core if malloc returned a
buffer containing only non-NUL bytes.
(init_tty): Do not allocate or free termcap buffers; the
struct does that for us now.
* termchar.h (TERMCAP_BUFFER_SIZE) [!TERMINFO]: New constant.
(struct tty_display_info): Define members termcap_term_buffer and
termcap_strings_buffer only if !TERMINFO, since terminfo doesn't
use them. Allocate them directly in struct rather than indirectly
via a pointer, to simplify init_tty.