Add arguments to mxb_assert_message

The macro can now be used to print runtime information in a printf-like
manner. This makes it easier to see exactly why an assertion has failed.

maxutils/maxbase/include/maxbase/assert.h

@@ -31,9 +31,11 @@ MXB_BEGIN_DECLS
             fprintf(stderr, "debug assert at %s:%d failed: %s\n", (char*)__FILE__, __LINE__, debug_expr); \
             raise(SIGABRT);}} while (false)
 
-#define mxb_assert_message(exp, message) \
+#define mxb_assert_message(exp, fmt, ...) \
     do {if (exp) {} else {     \
             const char* debug_expr = #exp; \
+            char message[1024]; \
+            snprintf(message, sizeof(message), fmt, ##__VA_ARGS__); \
             MXB_ERROR("debug assert at %s:%d failed: %s (%s)\n", \
                       (char*)__FILE__, \
                       __LINE__, \