Added printf format checking to logging function.

This also required changing the logging of "" at the
start of each test to appeal the otherwise-friendly
compiler.

test-tool/test_write12_flags.c

@@ -33,7 +33,7 @@ test_write12_flags(void)
 
 	CHECK_FOR_DATALOSS;
 
-	logging(LOG_VERBOSE, "");
+	logging(LOG_VERBOSE, LOG_BLANK_LINE);
 	logging(LOG_VERBOSE, "Test WRITE12 flags");
 
 	buf = malloc(block_size);