blktap2: initialise buf to NULL in img2qcow.c:main

blktap2: initialise buf to NULL in img2qcow.c:main

Gcc complains:

qcow2raw.c: In function ‘main’:
qcow2raw.c:387:17: error: ‘buf’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
    treq.buf     = buf;
                 ^

But at the point of that assignment, buf is a valid buffer allocated by
posix_memalign and filled in by read.

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>