Properly nul-terminate return values from g_convert()

Properly nul-terminate return values from g_convert()

The patch ensures that multibyte character sets are properly
nul-terminated. This is an issue because the documentation claims to
return a "nul-terminated" string and users of the API assume that means
a proper nul-termination in the resulting character set.

I looked at Pidgin and GStreamer code and found at least 3 cases where
this was happening.

This patch also reverts the documentation change from
5a633f82dbbce067c56fba0ee372fd341b76368d as that is now no longer
necessary.