Fix DecodeBase64 to reject truncated input

[jwnimmer-tri]

If the input doesn't have the proper number of encoding characters (a multiple of 4), return an empty result.

[jbeder]

Why?

Alternatively, what about throwing an exception since it's a failed conversion?

[jwnimmer-tri]

Why?

In general for parsers, it's important to detect and reject invalid input.

Alternatively, what about throwing an exception since it's a failed conversion?

The pre-existing logic in the function already returns the empty vector to indicate a conversion failure, e.g., for invalid characters:

yaml-cpp/src/binary.cpp

Lines 83 to 84 in 2e6383d

	if (d == 255)
	return ret_type();

I was trying to remain consistent with existing practice.

If you would prefer that all decode failures throw instead of returning empty, I can work on revising the patch?

[jwnimmer-tri]

Note that the call site already correctly interprets an empty return value as an error:

yaml-cpp/include/yaml-cpp/node/convert.h

Lines 462 to 464 in 2e6383d

	std::vector<unsigned char> data = DecodeBase64(node.Scalar());
	if (data.empty() && !node.Scalar().empty())
	return false;

[jbeder]

Ok sounds good