[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. -- best regards, Max, http://fairwaves.ru