Write past end of buffer in bson_decimal128_to_string

When libbson translates to JSON the following decimal 128 data:

00110000001100000011000000110000001100000011000000110000001100000011000000110000001100000011000000110000111111111111111110000000

It should stringify it as:

-1.036812917280316154812861194857272E-6015

Instead it stringifies it incorrectly as:

-1.0368129172803161548128611948572720E-6015

The trailing zero is out of spec (there should only be 34 decimal digits, the "0" makes 35) and causes libbson to write a byte past the end of its fixed-size string buffer.

This is low-impact: the byte past the end of a 43-byte buffer won't be used for anything by a compiler that word-aligns the following object.