CDRIVER-5681 Address potential buffer overflow when writing null terminator after decimal128 significand digits

[eramongodb]

Resolves CDRIVER-5681. See ticket for details.

[kevinAlbs]

Great catch. LGTM.

Suggest adding a regression test, in case bson_decimal128_to_string is modified again in the future.

[eramongodb]

After much effort trying (and failing) to craft a human-readable/easily-understandable test case for this bug, it is now unclear to me whether this "significand truncation" behavior is even the correct behavior to begin with. I am having great difficulty finding any detailed specification or reference for the decimal128 (BID) -> string conversion algorithm against which the results can be compared. Nevertheless, I added an untruncated_significand test case to hopefully make clear how the inclusion of the negative sign leads to the omission of the trailing 1 digit at the end due to length limitations.

[eramongodb]

Added annotations for values where inexact rounding errors may occur when attempting to convert the string representation to a Decimal128 value. On that note, added missing assertions of the return value of bson_decimal128_from_string*() and converted BSON_ASSERT -> ASSERT for better error messages on assertion failure.

[vector-of-bool]

LGTM