Fix another dangling pointer

Same problem as the previous commit: toStdString() returns a temporary,
and c_str() will return a pointer to internal data, freed at the end of
the statement. So get the pointer to be strcpy'ed in the same statement.

Changed to toUtf8() to be more explicit about the encoding and to avoid
std::string

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>