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>

tools/blktap2/drivers/img2qcow.c

@@ -166,7 +166,7 @@ int main(int argc, const char *argv[])
         int ret = -1, fd, len, err;
 	struct timeval timeout;
 	uint64_t i;
-	char *buf;
+	char *buf = NULL;
 	td_request_t treq;
         td_disk_info_t info;
         td_vbd_request_t* vreq;