Fix thread-safety issue in debug.c

Closes #203
diff --git a/tests/suites/test_suite_debug.function b/tests/suites/test_suite_debug.function
index 7db04e5..df34010 100644
--- a/tests/suites/test_suite_debug.function
+++ b/tests/suites/test_suite_debug.function
@@ -44,8 +44,8 @@
     debug_set_threshold( threshold );
     ssl_set_dbg(&ssl, string_debug, &buffer);
 
-    debug_print_msg( &ssl, level, file, line,
-                     debug_fmt("Text message, 2 == %d", 2 ) );
+    debug_print_msg_free( &ssl, level, file, line,
+                          debug_fmt("Text message, 2 == %d", 2 ) );
 
     TEST_ASSERT( strcmp( buffer.buf, result_str ) == 0 );
 }