Update security/stunnel to 5.51:

Version 5.51, 2019.04.04, urgency: MEDIUM

New features
Hexadecimal PSK keys are automatically converted to binary.
Session ticket support (requires OpenSSL 1.1.1 or later). "connect"
address persistence is currently unsupported with session tickets.
SMTP HELO before authentication (thx to Jacopo Giudici).
New "curves" option to control the list of elliptic curves in OpenSSL
1.1.0 and later.
New "ciphersuites" option to control the list of permitted TLS 1.3 ciphersuites.
Include file name and line number in OpenSSL errors.
Compatibility with the current OpenSSL 3.0.0-dev branch.
Better performance with SSL_set_read_ahead()/SSL_pending().
Bugfixes
Fixed PSKsecrets as a global option (thx to Teodor Robas).
Fixed a memory allocation bug (thx to matanfih).

(jklos)