SSL: removed logging of empty "(SSL:)" in ngx_ssl_error(). The "(SSL:)" snippet currently appears in logs when nginx code uses ngx_ssl_error() to log an error, but OpenSSL's error queue is empty. This can happen either because the error wasn't in fact from OpenSSL, or because OpenSSL did not indicate the error in the error queue for some reason. In particular, currently "(SSL:)" can be seen in errors at least in the following cases: - When SSL_write() fails due to a syscall error, "[info] ... SSL_write() failed (SSL:) (32: Broken pipe)...". - When loading a certificate with no data in it, "[emerg] PEM_read_bio_X509_AUX(...) failed (SSL:)". This can easily happen due to an additional empty line before the end line, so all lines of the certificate are interpreted as header lines. - When trying to configure an unknown curve, "[emerg] SSL_CTX_set1_curves_list("foo") failed (SSL:)". Likely there are other cases as well. With this change, "(SSL:...)" will be only added to the error message if there is something in the error queue. This is expected to make logs more readable in the above cases. Additionally, with this change it is now possible to use ngx_ssl_error() to log errors when some of the possible errors are not from OpenSSL and not expected to have anything in the error queue.