libnm: handle illegal characters in nm_utils_ssid_to_utf8() (rh #1243078)

libnm: handle illegal characters in nm_utils_ssid_to_utf8() (rh #1243078)

g_convert_with_fallback() will fail if the SSID contains characters that
are not legal in the source encoding, which, if $LANG is not set, will
be ASCII. If this happens, replace all non-ASCII and non-printable
characters with '?'. It is possible that nm_utils_ssid_to_utf8() will
now return an empty string (e.g., the source string is actually
big-endian UTF-16 and g_strcanon() stops on the first byte), but it will
not return NULL.

https://bugzilla.redhat.com/show_bug.cgi?id=1243078