JDK-8166339, Code conversion working behavior was changed for x-IBM834

[Xueming Shen]

Hi,

Please help review the change for JDK-8166339.

issue: https://bugs.openjdk.java.net/browse/JDK-8166339
webrev: http://cr.openjdk.java.net/~sherman/8166339/webrev

This is a regression triggered by

issue: https://bugs.openjdk.java.net/browse/JDK-8008386
webrev: http://cr.openjdk.java.net/~sherman/8008386/webrev

which updated the doublebyte decoder implementation to handle unmappable
bytes more "appropriately" ( malformed (1) or unmappable(2) ).

However in case of Cp834, which is a doublebyte-only charset, the unmappable
bytes should always be treated as double-byte pair, therefor always 
unmappable(2).

Thanks
Sherman