Reject out-of-range latitudes when doing CPR decoding.

If a CPR message with an undetected error is received this can produce out-of-range results for latitude.
e.g. even latitude of 78000, odd latitude of 0 produces a latitude index j=35 and rlat0 = 213.
Merge pull request #40 from mutability/discard-bad-cpr-latitudes

Reject out-of-range latitudes when doing CPR decoding.