libnm: fix unterminated NUL string when parsing UDev properties

This can result in trailing garbage (which fails UTF-8 validation
checks) or even worse, in read-out-of-bounds.

Fixes: 6808bf8195

https://bugzilla.redhat.com/show_bug.cgi?id=1443114
https://bugzilla.redhat.com/show_bug.cgi?id=1451160
https://bugzilla.redhat.com/show_bug.cgi?id=1451286
(cherry picked from commit 9594ee6e69)

shared/nm-utils/nm-udev-utils.c

@@ -89,6 +89,7 @@ nm_udev_utils_property_decode (const char *uproperty, char **to_free)
 		return uproperty;
 	}
 
+	*n++ = '\0';
 	return (*to_free = unescaped);
 }