aio: Avoid spurious coverity warning

PgAioResult.result is never accessed in the relevant path, but coverity
complains about an uninitialized access anyway. So just zero-initialize the
whole thing.  While at it, reduce the scope of the variable.

Reported-by: Ranier Vilela <ranier.vf@gmail.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CAEudQApsKqd-s+fsUQ0OmxJAMHmBSXxrAz3dCs+uvqb3iRtjSw@mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -6868,8 +6868,6 @@ buffer_readv_complete_one(PgAioTargetData *td, uint8 buf_off, Buffer buffer,
 	/* Check for garbage data. */
 	if (!failed)
 	{
-		PgAioResult result_one;
-
 		if (!PageIsVerified((Page) bufdata, tag.blockNum, piv_flags,
 							failed_checksum))
 		{
@@ -6904,6 +6902,8 @@ buffer_readv_complete_one(PgAioTargetData *td, uint8 buf_off, Buffer buffer,
 		 */
 		if (*buffer_invalid || *failed_checksum || *zeroed_buffer)
 		{
+			PgAioResult result_one = {0};
+
 			buffer_readv_encode_error(&result_one, is_temp,
 									  *zeroed_buffer,
 									  *ignored_checksum,