[PATCH] Improve hex parser

Hello.

Current implementation of osmo_hexparse have couple of limitations (which are undocumented btw):

* it refuses to parse odd-length numbers (e. g. 7abdf) * it fails while parsing 0x prefix (e. g. 0xdead)

Both could be worked around by upper-layer code of course but since both cases above are perfectly valid hexadecimal numbers I think they should be handled by osmo_hexparse internally without complicating life of library users.

Attached patch does just that. Please review and merge if it feels OK.