Check if object creation failed, happens for abstract classes.

[cypof]

Not sure if the exception should be cleared. JavaCPP currently segfaults a bit later, I guess it would be better to raise the exception on the Java side.

[saudet]

AllocObject() returns NULL when it fails, and it usually gets called just before returning to Java (but not always, might have to study this a bit further), so we might as well let it propagate the exception back to Java. I fixed a couple of missing NULL checks in the commit above, and now when calling FloatLayer.layer_by_name(), for example, we get a proper stack trace like this:

java.lang.InstantiationException: org.bytedeco.javacpp.caffe$FloatLayer
    at org.bytedeco.javacpp.caffe$FloatNet.layer_by_name(Native Method)
    at caffe$3.command(caffe.java:238)
    at caffe.main(caffe.java:392)

Is this satisfactory?

[cypof]

Yes that's great, a java exception is better than just a message, thanks