xkb_keysym_to_utf8() requires output buffer size to be at least 7, but it should be 5

[LocalSpook]

As I understand it, a size of 7 was needed when UTF-8 sequences could be up to 6 bytes long. Now that they're limited to 4, xkb_keysym_to_utf8()'s requirement should be reduced accordingly.

[wismill]

I agree. We just need max 4 bytes for the character plus 1 for the null character. I added a test for this in #414.

[wismill]

A bit of history:

• rfc2044 (1996, up to 6 bytes), superseded by rfc2279 (1998, up to 6 bytes), superseded by rfc3629 (2003, up to 4 bytes).
• Wikipedia: UTF-8 History