Comment unused TestStringClass() and fix build with Clang 6

code/splines/util_str.cpp:477:11: error: reinterpret_cast from 'nullptr_t' to
'const char *' is not allowed
        idStr e( reinterpret_cast<const char *>( NULL ) );
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
code/splines/util_str.cpp:491:10: error: reinterpret_cast from 'nullptr_t' to
'const char *' is not allowed
        a = c + reinterpret_cast<const char *>( NULL );
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
code/splines/util_str.cpp:494:6: error: reinterpret_cast from 'nullptr_t' to
'const char *' is not allowed
        a = reinterpret_cast<const char *>( NULL ) + d;
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
code/splines/util_str.cpp:498:7: error: reinterpret_cast from 'nullptr_t' to
'const char *' is not allowed
        a += reinterpret_cast<const char *>( NULL );
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--- MP/code/splines/util_str.cpp.orig	2018-01-08 12:19:28 UTC
+++ MP/code/splines/util_str.cpp
@@ -454,6 +454,7 @@ void idStr::snprintf
 #pragma warning(disable : 4189) // local variable is initialized but not referenced
 #endif
 
+#if 0
 /*
 =================
 TestStringClass
@@ -526,6 +527,7 @@ void TestStringClass
 
 	a[1] = '1';                 // a.data = "t1st", b.data = "test"
 }
+#endif
 
 #ifdef _MSC_VER
 #pragma warning(default : 4189) // local variable is initialized but not referenced
--- SP/code/splines/util_str.cpp.orig	2018-01-08 12:19:44 UTC
+++ SP/code/splines/util_str.cpp
@@ -454,6 +454,7 @@ void idStr::snprintf
 #pragma warning(disable : 4189) // local variable is initialized but not referenced
 #endif
 
+#if 0
 /*
 =================
 TestStringClass
@@ -526,6 +527,7 @@ void TestStringClass
 
 	a[1] = '1';                 // a.data = "t1st", b.data = "test"
 }
+#endif
 
 #ifdef _MSC_VER
 #pragma warning(default : 4189) // local variable is initialized but not referenced