fix: prevent overflow in minmea_scan 's' format specifier

[dwilliams27]

Need to add length check in minmea_scan format specifier, otherwise could have buffer overflow

[hraftery]

Just a outsider comment - what makes MINMEA_MAX_SENTENCE_LENGTH special? I agree there's potential overflow, but I would have thought sentence is more likely to be at least that size, than buf. In other words, what's the chances this prevents buf being overflowed, given we don't know how big it is?

Or maybe the better question is, how is minmea_scan used with the s format option? I must admit I don't actually know.

Isn't this just as likely to overflow as strcpy() ?