Silence clang missing-field-initializers warning

When libiscsi is built with clang several spurious
missing-field-initializers warnings like the following are reported:

test_async_write.c:61:45: error: missing field 'completed' initializer
[-Werror,-Wmissing-field-initializers]
	struct tests_async_write_state state = { 0 };

These appear to be due to clang being overzealous in its checking
because the code is legal C (see
https://llvm.org/bugs/show_bug.cgi?id=21689 ) but it causes compilation
to be aborted. Workaround the problem by specifying a 0 or NULL for each
structure member.

test-tool/test_async_write.c

@@ -58,7 +58,7 @@ void
 test_async_write(void)
 {
 	int i, ret;
-	struct tests_async_write_state state = { 0 };
+	struct tests_async_write_state state = { 0, 0, 0 };
 	int blocksize = 512;
 	int blocks_per_io = 8;
 	int num_ios = 1000;