Testbench: Propagate fail from topology parse load widget

This patch avoids the testbench to crash when file read or write
file open was not successful. E.g. invoking testbench with
incorrect input file name caused a segfault.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

tools/testbench/topology.c

@@ -555,5 +555,5 @@ finish:
 
 	free(temp_comp_list);
 	fclose(file);
-	return 0;
+	return ret;
 }