BUG/MEDIUM: ssl_sample: fix segfault for srv samples on invalid request

BUG/MEDIUM: ssl_sample: fix segfault for srv samples on invalid request

Some ssl samples cause a segfault when the stream is not instantiated,
for example during an invalid HTTP request. A new check is added to
prevent the stream dereferencing if NULL.

This is the list of the affected samples :
- ssl_s_chain_der
- ssl_s_der
- ssl_s_i_dn
- ssl_s_key_alg
- ssl_s_notafter
- ssl_s_notbefore
- ssl_s_s_dn
- ssl_s_serial
- ssl_s_sha1
- ssl_s_sig_alg
- ssl_s_version

This bug can be reproduced easily by using one of these samples in a
log-format string. Emit an invalid HTTP request with an HTTP client to
trigger the crash.

This bug has been reported in redmine issue 3913.

This must be backported up to 2.2.