GUIX binary resource loader reads past the end of a resource, because no length for it ever reaches the loader

`gx_binres_theme_load()` and `gx_binres_font_load()` take the resource as a bare `GX_UBYTE *` with no length, and `GX_BINRES_DATA_INFO` has no field to carry one. Every read is therefore performed at an offset the file itself chose, against a buffer whose extent the loader does not know. A resource that declares more data than it contains is read past its end, and an offset stored in the file is turned into a pointer without being bounded.

This is a distinct defect from GHSA-hgcj-22cg-4hfv, which was an underflowed glyph count. It needs no underflow and no wrapped multiply: a perfectly well-formed, narrow glyph range reaches it.