gbase64: Convert a precondition from g_error() to g_return_val_if_fail()
gbase64: Convert a precondition from g_error() to g_return_val_if_fail()

The caller needs to check this themselves in any case, so we might as
well at least follow convention in defining the precondition.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib