tests/virtio-9p: add terminating null in v9fs_string_read()

The 9p protocol sends strings in general without null termination
over the wire. However for future use of this functions it is
beneficial for the delivered string to be null terminated though
for being able to use the string with standard C functions which
often rely on strings being null terminated.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <52c84e2ce3bcafc2a38eed13b8c8e23bc1a8ecb9.1579567019.git.qemu_oss@crudebyte.com>
Signed-off-by: Greg Kurz <groug@kaod.org>

tests/qtest/virtio-9p-test.c

@@ -130,8 +130,9 @@ static void v9fs_string_read(P9Req *req, uint16_t *len, char **string)
         *len = local_len;
     }
     if (string) {
-        *string = g_malloc(local_len);
+        *string = g_malloc(local_len + 1);
         v9fs_memread(req, *string, local_len);
+        (*string)[local_len] = 0;
     } else {
         v9fs_memskip(req, local_len);
     }