dtls: check length before parsing client identity
dtls: check length before parsing client identity

instead of just taking the first 32 bytes, check the length and reject
it if it is too long.

Signed-off-by: Hauke Mehrtens <435ddd46dc66c007a1fa20144c823d37e0d23436@hauke-m.de>

fvdnabee/tinydtls,OlegHahm/tinydtls,jvermillard/tinydtls,fvdnabee/tinydtls,nmacs/tinydtls,yunus/TinyDTLS-WebID,fvdnabee/tinydtls,yunus/TinyDTLS-WebID,nmacs/tinydtls,OlegHahm/tinydtls,OlegHahm/tinydtls,jvermillard/tinydtls,thecodemaiden/tinydtls-fork,jvermillard/tinydtls,spark/tinydtls,thecodemaiden/tinydtls-fork,yunus/TinyDTLS-WebID,spark/tinydtls,nmacs/tinydtls,nmacs/tinydtls,spark/tinydtls,thecodemaiden/tinydtls-fork