Fix warning for zero variadic macro arguments. (#2151)

When consuming dlib headers and building using gcc/clang with flags
'-Werror -Wpedantic', any inclusion involving DLIB_CASSERT triggers
a compilation error: ISO C++11 requires at least one argument for the
"..." in a variadic macro

Co-authored-by: Samuel Aldana <samuel.aldana@cognex.com>

dlib/assert.h

@@ -160,7 +160,7 @@ namespace dlib
 #define DLIBM_CASSERT_1_ARGS(exp)              DLIBM_CASSERT(exp,"")
 #define DLIBM_CASSERT_2_ARGS(exp,message)      DLIBM_CASSERT(exp,message)
 #define DLIBM_GET_3TH_ARG(arg1, arg2, arg3, ...) arg3
-#define DLIBM_CASSERT_CHOOSER(...) DLIB_WORKAROUND_VISUAL_STUDIO_BUGS(DLIBM_GET_3TH_ARG(__VA_ARGS__,  DLIBM_CASSERT_2_ARGS, DLIBM_CASSERT_1_ARGS))
+#define DLIBM_CASSERT_CHOOSER(...) DLIB_WORKAROUND_VISUAL_STUDIO_BUGS(DLIBM_GET_3TH_ARG(__VA_ARGS__,  DLIBM_CASSERT_2_ARGS, DLIBM_CASSERT_1_ARGS, NEVER_USED))
 #define DLIB_CASSERT(...) DLIB_WORKAROUND_VISUAL_STUDIO_BUGS(DLIBM_CASSERT_CHOOSER(__VA_ARGS__)(__VA_ARGS__))