[master] Change default value of rcode_ to -1

This is because the tests which come later check if it has been set
to 0.

src/bin/dhcp4/tests/dhcp4_srv_unittest.cc

@@ -194,7 +194,7 @@ public:
     /// Initializes common objects used in many tests.
     /// Also sets up initial configuration in CfgMgr.
     Dhcpv4SrvTest() :
-        rcode_(0)
+        rcode_(-1)
     {
         subnet_ = Subnet4Ptr(new Subnet4(IOAddress("192.0.2.0"), 24, 1000,
                                          2000, 3000));