initialize Jsonnet::vm_ field to nullptr

The Jsonnet class constructor doesn't set up the VM; this is deferred
until the user calls init(). However it's important for the vm_ field to
itself be initialised to a known value. The Jsonnet destructor will
attempt to destroy the VM only if the vm_ field is not null.

cpp/libjsonnet++.cpp

@@ -17,7 +17,7 @@ limitations under the License.
 #include "libjsonnet++.h"
 
 namespace jsonnet {
-Jsonnet::Jsonnet() {}
+Jsonnet::Jsonnet(): vm_(nullptr) {}
 
 Jsonnet::~Jsonnet()
 {