block.c: fix warning with _FORTIFY_SOURCE

  CC    block.o
cc1: warnings being treated as errors
block.c: In function 'bdrv_open2':
block.c:400: error: ignoring return value of 'realpath', declared with attribute warn_unused_result
Signed-off-by: NKirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

block.c

@@ -396,8 +396,8 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
         if (is_protocol)
             snprintf(backing_filename, sizeof(backing_filename),
                      "%s", filename);
-        else
-            realpath(filename, backing_filename);
+        else if (!realpath(filename, backing_filename))
+            return -errno;
 
         bdrv_qcow2 = bdrv_find_format("qcow2");
         options = parse_option_parameters("", bdrv_qcow2->create_options, NULL);