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>
1 file changed