ecc: make signature check return negative number in error case.
ecc: make signature check return negative number in error case.

dtls excepted a negative number now, this was a security problem.

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

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