Do not eat ESC character if control string is not properly terminated.

Do not eat ESC character if control string is not properly terminated.

Currently tputc handles the case of too long control string waiting for
the end of control string.

Another case is when there is ESC character is encountered but is not
followed by '\\'.  In this case st stops processing control string,
but ESC character is ignored.

After this patch st processes ESC characters in control strings properly.

Test case:
printf '\e]0;abc\e[1mBOLD\e[0m'

Also ^[\ is actually processed in the code that handles ST.
According to ECMA-048 ST stands for STRING TERMINATOR and is used to
close control strings.