Wrong coding style

[watiko]

v7.4.785...v7.4.790

• 7.4.786 : it is not possible for a plugin to adjust to a changed setting
• 7.4.787 : (after 7.4.786) snprintf() isn't available everywhere
• 7.4.788 : (after 7.4.786) can't build without the crypt feature
• 7.4.789 : (after 7.4.788) using freed memory and crash
• 7.4.790 : (after 7.4.786) test fails without the autochdir feature

There are still mixed sprintf and vim_snprintf cases.
https://github.com/vim/vim/blob/v7.4.943/src/option.c#L4979
https://github.com/vim/vim/blob/v7.4.943/src/option.c#L5808

We does not use the last allocated byte of buf_old and buf_new.
EDIT: And Should we use NUMBUFLEN?

    char_u buf_old[11], buf_new[11], buf_type[7];
    vim_snprintf((char *)buf_old, 10, "%ld", old_value);
    vim_snprintf((char *)buf_new, 10, "%ld", value);

https://github.com/vim/vim/blob/v7.4.943/src/option.c#L8957-L8959

EDIT2
If errmsg equals NULL, saved_origval won't be freed (memory leaking).
https://github.com/vim/vim/blob/v7.4.943/src/option.c#L4955-L4991

[watiko]

a988496 fixes the memory leak.