Made DLIB_ASSERT turn into an empty {} when disabled instead of turning

into nothing.  This way if you have a DLIB_ASSERT inside an if statement
it doesn't change anything about the code if it's disabled.

dlib/assert.h

@@ -106,7 +106,7 @@ namespace dlib
     #define DLIB_ASSERT(_exp,_message) DLIB_CASSERT(_exp,_message)
     #define DLIB_IF_ASSERT(exp) exp
 #else
-    #define DLIB_ASSERT(_exp,_message)
+    #define DLIB_ASSERT(_exp,_message) {}
     #define DLIB_IF_ASSERT(exp) 
 #endif