Fix signedness issue with snprintf

[durka]

AP_InternalError::errors_as_string casts the return value of snprintf, which is signed int, to unsigned size_t. So the comparison <= 0 is effectively == 0, ignoring errors.

I don't think snprintf actually returns an error unless the format string is wrong, which it isn't, so this is probably mostly academic. I don't believe the == 0 case is hit anyway, since the loop exits earlier by checking buffer_used >= len.

Also included a micro-optimization to avoid a conditional in the loop.

Note: I am employed by Exyn Technologies

[tridge]

@durka thanks!
minor thing, our code style requires the commit to start with the subsystem, so:
AP_InternalError: fix signedness issue with snprintf
I can fix your PR if you don't know how to reword a commit

[durka]

updated the commit message

[peterbarker]

Merged, thanks!