dtls: send no alert when we do not have a peer
dtls: send no alert when we do not have a peer

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

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