[PATCH 08 of 18] tools/blktap2: fix build errors caused by Werror in tdqcow_get_parent_id

[Olaf Hering <olaf@aepfle.de>]

# HG changeset patch
# User Olaf Hering <olaf@aepfle.de>
# Date 1333397540 -7200
# Node ID e9b9e8254311fb61930ab5a954e777928466c607
# Parent  acb561c6d4d8ebee95ad0e7007f99a2d22dbaa34
tools/blktap2: fix build errors caused by Werror in tdqcow_get_parent_id

-O2 -Wall -Werror triggers these warnings:

block-qcow.c: In function 'tdqcow_get_parent_id':
block-qcow.c:1457:17: warning: 'type' may be used uninitialized in this function [-Wuninitialized]

The compiler can not know that open() writes to errno so it has to
assume that errno can be zero. Use assert as hint for gcc.

v2:
 - add assert() as suggested by IanJ

Signed-off-by: Olaf Hering <olaf@aepfle.de>

diff -r acb561c6d4d8 -r e9b9e8254311 tools/blktap2/drivers/block-qcow.c
--- a/tools/blktap2/drivers/block-qcow.c
+++ b/tools/blktap2/drivers/block-qcow.c
@@ -34,6 +34,7 @@
 #include <inttypes.h>
 #include <libaio.h>
 #include <limits.h>
+#include <assert.h>
 #include "bswap.h"
 #include "aes.h"
 #include "md5.h"
@@ -1407,8 +1408,10 @@ tdqcow_get_image_type(const char *file, 
 	QCowHeader header;
 
 	fd = open(file, O_RDONLY);
-	if (fd == -1)
+	if (fd == -1) {
+		assert(errno);
 		return -errno;
+	}
 
 	size = read(fd, &header, sizeof(header));
 	close(fd);