GVariant: Better handling of invalid UTF-8

GVariant: Better handling of invalid UTF-8

g_variant_new_string() hits a g_return_if_fail() when given invalid
UTF-8.  That's certainly the right thing to do, but
g_variant_builder_add() uses this function internally and crashes when
it returns NULL due to the user passing a non-utf8 string.

We can protect the internal code by returning "[Invalid UTF-8]" in this
case while also making the problem easier to debug (and less fatal).

Closes #632631.