RFR 8014217: Base64.getXDecoder().wrap(...).read() doesn't throw exception for an incorrect number of padding chars in the final unit

[Xueming Shen]

Hi,

Please help review the change for handling a "malformed base64 stream" corner case.
The latest spec has been updated/clarified as

"If there is padding character present in the final unit, the correct number of padding
character(s) must be present, otherwise IllegalArgumentException is thrown during
decoding."

But the stream decoding implementation has not been updated accordingly. The change
is to throw the IOE if the padding character "=" is missing from the xx== pattern.

http://cr.openjdk.java.net/~sherman/8014217/webrev/

Thanks!
Sherman