added bounds checks for invalid datetime decoding
added bounds checks for invalid datetime decoding

If we neglect to set the FTIME parameter during our
upload procedure, we'll get a new file on FlashAir with
a date of 1980-00-00 00:00, which is, of course, invalid.
When `arrow` attempts to decode that date, it'll crash.
We just need to be sure that the month and day are > 0.

TadLeonard/tfatool