Now also fix dia_assert_true() declaration and use.

debug.c:39:15: warning: format string is not a string literal [-Wformat-nonliteral]
    g_vprintf(format, args);
              ^~~~~~

And followup warnings of the fix:

object.c:1109:49: warning: more '%' conversions than data arguments [-Wformat]
                    "%s: Object %p has illegal type name %p (%s)\n",
                                                             ~^
object.c:1118:45: warning: format specifies type 'char *' but the argument has
      type 'const DiaObject *' (aka 'const struct _DiaObject *') [-Wformat]
                      "%s: Object %p has null handles\n", obj);
                       ~~                                 ^~~
object.c:1118:22: warning: more '%' conversions than data arguments [-Wformat]
                      "%s: Object %p has null handles\n", obj);
                                  ~^