Fix RouteBase string formatting bug

When correcting conversion issues in RouteBase a to_string bug was
introduced which caused some characters to be escaped when inserted
to the string, for example a prefix_len of 0 would render as "\0"
rather than inserting '0'. The std::ios::binary flag does not seem
to prevent this for std::ostringstream so I have cast the data member
up to uint16_t which should be safe, and solves the issue.

Added a unit test to demonstrate the issue. Old code output was
"0.0.0.0/\0", now outputs "0.0.0.0/0" as expected.

Signed-off-by: Charlie Vigue <charlie.vigue@openvpn.com>

openvpn/tun/builder/capture.hpp

@@ -150,7 +150,7 @@ class TunBuilderCapture : public TunBuilderBase, public RC<thread_unsafe_refcoun
         std::string to_string() const
         {
             std::ostringstream os;
-            os << address << '/' << prefix_length;
+            os << address << '/' << static_cast<uint16_t>(prefix_length);
             if (!gateway.empty())
                 os << " -> " << gateway;
             if (metric >= 0)

test/unittests/CMakeLists.txt

@@ -67,6 +67,7 @@ add_executable(coreUnitTests
         test_streq.cpp
         test_time.cpp
         test_typeindex.cpp
+        test_tun_builder.cpp
         test_userpass.cpp
         test_validatecreds.cpp
         test_weak.cpp

test/unittests/test_tun_builder.cpp

@@ -0,0 +1,14 @@
+
+#include "test_common.h"
+
+#include <openvpn/tun/builder/capture.hpp>
+
+TEST(tun_builder, to_string_zero)
+{
+    auto rb = openvpn::TunBuilderCapture::RouteBase();
+
+    rb.address = "0.0.0.0";
+    rb.prefix_length = 0;
+
+    EXPECT_EQ(rb.to_string(), "0.0.0.0/0");
+}