SECURITY: Don't miss the very last byte when reading long lines from -H

SECURITY: Don't miss the very last byte when reading long lines from -H

Credits: Qualys

    2/ In src/spool_in.c:

     462   while (  (len = Ustrlen(big_buffer)) == big_buffer_size-1
     463         && big_buffer[len-1] != '\n'
     464         )
     465     {   /* buffer not big enough for line; certs make this possible */
     466     uschar * buf;
     467     if (big_buffer_size >= BIG_BUFFER_SIZE*4) goto SPOOL_READ_ERROR;
     468     buf = store_get_perm(big_buffer_size *= 2, FALSE);
     469     memcpy(buf, big_buffer, --len);

    The --len in memcpy() chops off a useful byte (we know for sure that
    big_buffer[len-1] is not a '\n' because we entered the while loop).

(cherry picked from commit 58454ea01c2e817481770954edf09ad82f3cd417)