lib/utils: Check for invalid UTF-8 in filenames

lib/utils: Check for invalid UTF-8 in filenames

In case a filename contains invalid UTF-8 characters, libostree will
pass it to g_variant_builder_add() in create_tree_variant_from_hashes()
anyway, which leads to a critical warning from glib and an invalid
commit. This commit makes ostree print a useful error and exit instead.

Closes: #1271
Approved by: cgwalters