nic_router: safe pointers that are valid and const

Add safe pointer constructor that takes a reference as argument to enable the
use of the safe pointer wrapper as const object.

Ref #2670

repos/os/src/server/nic_router/pointer.h

@@ -33,6 +33,8 @@ class Net::Pointer
 
 		Pointer() : _ptr(nullptr) { }
 
+		Pointer(T &ref) : _ptr(&ref) { }
+
 		T &deref() const
 		{
 			if (_ptr == nullptr) {