ssl_cipher_process_rulestr: don't read outside rule_str buffer

ssl_cipher_process_rulestr: don't read outside rule_str buffer

If rule_str ended in a "-", "l" was incremented one byte past the
end of the buffer.  This resulted in an out-of-bounds read when "l"
is dereferenced at the end of the loop.  It is safest to just return
early in this case since the condition occurs inside a nested loop.

CLA: trivial

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19166)