blktap2: initialise buf in vhd_util_check_footer

blktap2: initialise buf in vhd_util_check_footer

Gcc complains:

vhd-util-check.c: In function ‘vhd_util_check_footer’:
vhd-util-check.c:413:2: error: ‘buf’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  memcpy(&backup, buf, sizeof(backup));

In fact buf is initialised a few lines above.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>