DPIC: reset diffstate_buffer to nullptr after free (#278)

DPI-C functions are using the diffstate_buffer to check whether
the corresponding data structures have been initialized. However,
we are not resetting it to nullptr after free. If the simulation
environment is reset manually and restarted, then this pointer
will be a dangling pointer and corrupt the next DPI-C calls.

src/main/scala/DPIC.scala

@@ -273,11 +273,13 @@ object DPIC {
          |    diffstate_buffer[i] = new DPICBuffer;
          |  }
          |}
+         |
          |void diffstate_buffer_free() {
          |  for (int i = 0; i < NUM_CORES; i++) {
          |    delete diffstate_buffer[i];
          |  }
          |  delete[] diffstate_buffer;
+         |  diffstate_buffer = nullptr;
          |}
       """.stripMargin
     interfaceCpp += interfaces.map(_._3).mkString("")