[nicl] Be more tolerant to zero-length

[Henry Jen]

Hi,

Please review the webrev[1] fixes a couple places where zero-length data will cause issues. Obvious example is that Scope.toCString(“”) chokes up is really not friendly, have to check for length by caller is too much burden IMHO.

A few function aid converting Java type in Scope is added, also added a Pointer.asDirectBuffer method, these are real use case correspond to JNI NewDirectByteBuffer.

Cheers,
Henry

[1] http://cr.openjdk.java.net/~henryjen/panama/ZeroLength/webrev/