conf-parser: fix wrong argument given to log_syntax_invalid_utf8

conf-parser: fix wrong argument given to log_syntax_invalid_utf8

The condition is on "word", hence we give word instead of rvalue.

An assert would be triggered if !utf8_is_valid(word) is true and
rvalue == NULL, since log_syntax_invalid_utf8 calls utf8_escape_invalid
which calls assert(str).

A test case has been added to test with valid and invalid utf8.