dtls: check identity length in client key exchange
dtls: check identity length in client key exchange

Before sending a client identity check if we have enough space for it.

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

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