test/librbd/fsx.c: fix gcc warning

Initialize pointer with NULL before call posix_memalign() to fix
gcc warning:

test/librbd/fsx.c:1402:13: warning: ‘temp_buf’ may be used
 uninitialized in this function [-Wmaybe-uninitialized]
  if ((ret = ops->read(&cur_ctx, 0, file_info.st_size, temp_buf)) < 0) {
             ^
test/librbd/fsx.c:1398:13: warning: ‘good_buf’ may be used
 uninitialized in this function [-Wmaybe-uninitialized]
  if ((ret = pread(fd, good_buf, file_info.st_size, 0)) < 0) {

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>