RMC parsing should reject sentences with data valid False

[peterhinch]

When power is first applied to the MTK3339 chip it emits RMC messages with gps_segments[2] != 'A'. The 'A' value only appears once a satellite fix has been achieved.

Unfortunately those invalid messages can have invalid date/time contents. These can pass subsequent tests, yielding an incorrect timestamp. In my opinion the 'A' value should be checked at the start of parsing, and the sentence rejected if it is invalid.

[Jollyjohn]

@peterhinch Good point. The NEMA standard for RMC is that this character shall be either A=data valid or V=data not valid. I agree that "A" should let the sentence be accepted and all other values cause rejection.