Revert last commit. The example packed message with the corrupted DateTime

was not a recoverable message header (e.g. the toUserName field was *not*
at offset 0x22):

xx 02 00 01 00 01 00 6A - 02 FE 01 00 00 00 00 30   : .......j.......0
34 20 46 65 62 20 31 31 - 39 20 20 32 30 3A 32 36   : 4 Feb 119  20:26
3A 33 32 04 00 4B 75 72 - 74 20 57 65 69 73 6B 65   : :32..Kurt Weiske

So restore the DateTime field validation: the 20th byte must be null.