lib: rsa: fix padding_pss_verify

Check the that the hash length is shorter than the message length. This
avoids:

    ./tools/../lib/rsa/rsa-verify.c:275:11: warning:
    ‘*db’ may be used uninitialized [-Wmaybe-uninitialized]
      275 |         db[0] &= 0xff >> leftmost_bits;

Fixes: 061daa0b ("rsa: add support of padding pss")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>