sim: common: add basic model assert

If the configured in default doesn't match a known value, throw an
assertion failure rather than segfaulting deeper down.

sim/common/ChangeLog

@@ -1,3 +1,7 @@
+2015-06-21  Mike Frysinger  <vapier@gentoo.org>
+
+	* sim-model.c (sim_model_init): Assert model is not NULL.
+
 2015-06-21  Mike Frysinger  <vapier@gentoo.org>
 
 	* sim-types.h: Move SIM_TYPES_H define to top of file.  Include

sim/common/sim-model.c

@@ -204,6 +204,7 @@ sim_model_init (SIM_DESC sd)
     {
       /* Set the default model.  */
       const MODEL *model = sim_model_lookup (WITH_DEFAULT_MODEL);
+      SIM_ASSERT (model != NULL);
       sim_model_set (sd, NULL, model);
     }