RFR JDK-8166258: Unexpected code conversion by HKSCS converters

[Xueming Shen]

Hi

Please help review the change for #8166258
issue: https://bugs.openjdk.java.net/browse/JDK-8166258
webrev: http://cr.openjdk.java.net/~sherman/8166258/webrev/

This is an overlook in the code that generates the c2b (encoding) mapping
table from the b2c data. \ufffd is the special code point that used to 
indicate
"no mapping" from a native code point to unicode code point. It's wrong to
try to generate a c2b mapping entry from \ufffd.

Thanks,
Sherman