base64dec: skip non-printable characters like \r\n

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Non-printable characters, such as line breaks, in a base64 encoded
string violate the "string length must be a multiple of four" rule.

This patch pads the result buffer by one extra unit of four bytes,
and skips over non-printable characters found in the input string.

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Gitea Version: 1.28.0+dev-505-g31b4d79a84