systemc: Make the report handler handle null messages.

It was checking the first character of the message for a null byte, but
not whether the message string pointer itself was null.

Change-Id: Iddef1e22c35b55c8c898670576ab416dd1023d7c
Reviewed-on: https://gem5-review.googlesource.com/12252
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/utils/sc_report_handler.cc

@@ -413,7 +413,7 @@ sc_report_compose_message(const sc_report &report)
     str << report.get_msg_type();
 
     const char *msg = report.get_msg();
-    if (msg[0])
+    if (msg && msg[0])
         str << ": " << msg;
 
     if (report.get_severity() > SC_INFO) {