RFR: 8204531: Remove unused chars following '\0'

[Yasumasa Suenaga]

Hi all,

Please review this change:

  webrev:
    http://cr.openjdk.java.net/~ysuenaga/JDK-8204531/webrev.00/

  JBS:
    https://bugs.openjdk.java.net/browse/JDK-8204531


We can use `jhsdb jsnap` to check all PerfData.
String values in PerfData are defined as jbyte array, but JSnap cannot handle 
it well as following:

```
$ jhsdb jsnap --pid 28542 --all | less

sun.gc.cause=No 
GC^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@
```

You can see this value via `less` and `vim` on Linux. `^@` shows it is 
non-ascii character.
PerfDataEntry has null-terminated C string. So we should restore as it in Java 
layer.


Thanks,

Yasumasa