Merge pull request #119 from LORD-MicroStrain/fix/packet

Fix packet writing field issue.

src/cpp/mip/mip_packet.hpp

@@ -124,7 +124,7 @@ class PacketView : public C::mip_packet_view
             if(ok)
                 ok &= C::mip_packet_update_last_field_length(&m_packet, basePointer(), (uint8_t) usedLength()) >= 0;
 
-            if(!ok)
+            if(!ok && basePointer())
                 C::mip_packet_cancel_last_field(&m_packet, basePointer());
 
             return ok;