Avoid JVM crash ;) when user asks for CPU/OpenCL mode on a platform that does...

Avoid JVM crash ;) when user asks for CPU/OpenCL mode on a platform that does not support CPU mode (NVidia).

com.amd.aparapi/src/java/com/amd/aparapi/KernelRunner.java

@@ -1381,6 +1381,9 @@ class KernelRunner{
                   } else {
                      // We fetch the first CPU device 
                      openCLDevice = (OpenCLDevice) OpenCLDevice.firstCPU();
+                     if (openCLDevice == null){
+                        return warnFallBackAndExecute(_entrypointName, _range, _passes, "CPU request can't be honored not CPU device");
+                     }
                   }
                } else {
                   if (openCLDevice.getType() == Device.TYPE.GPU) {