openpgp: Change the hex parsing to accept odd number of nibbles.
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This came up as the new leak tests use our hex parsing functions to parse /proc/self/maps and apparently Linux will drop leading zeros from addresses.
- Fix this by allowing these functions to operate on an odd number of nibbles. I see no reason no reason not to do that, except for the fact that we don't want to establish that it is okay to drop leading zeros from key IDs and fingerprints, hence I preserved the behavior of parsing key IDs and fingerprints.