Check it's an SSL request before dereferencing the ssl field of a connection

Note the comment in the test file about the difficulty of integrating a meaningful and automatic regression test in the testsuite.

Fix issue #26 (closed)

t/ssl_log_voms_plain_http.t

+
+# This test is always successful because, for some reason (a bug?)
+# the error.log as seen in Test::Nginx doesn't contain the entries for
+# the master process, although they are evailable in the actual file.
+# As a consequence the no_error_log check is always satisfied,
+# even if the segmentation fault were present
+
+use Test::Nginx::Socket 'no_plan';
+
+master_on();
+
+run_tests();
+
+__DATA__
+
+=== TEST 1: SSL server, logging a VOMS variabile, but plain HTTP request
+--- main_config
+    env X509_VOMS_DIR=t/vomsdir;
+--- http_config
+    log_format voms '$remote_addr - $remote_user [$time_local] '
+                    '"$request" $status $body_bytes_sent '
+                    '"$http_referer" "$http_user_agent" '
+                    '[$voms_user]';
+    server {
+        error_log logs/error.log debug;
+        access_log logs/access.log voms;
+        listen 8443 ssl;
+        ssl_certificate ../../certs/nginx_voms_example.cert.pem;
+        ssl_certificate_key ../../certs/nginx_voms_example.key.pem;
+        ssl_client_certificate ../../trust-anchors/igi-test-ca.pem;
+        ssl_verify_depth 10;
+	    location = / {
+            default_type text/plain;
+            echo $voms_user;
+        }
+    }
+--- config
+    location = /lua {
+        error_log logs/error-proxy.log debug;
+        access_log logs/access-proxy.log;
+        content_by_lua_block {
+            local sock = ngx.socket.tcp()
+            local ok, err = sock:connect("127.0.0.1", 8443)
+            if not ok then
+                ngx.say("failed to connect to upstream: ", err)
+                return
+            end
+            ngx.say("successfully connected to upstream!")
+            sock:send("G")
+            sock:close()
+        }
+    }
+--- request
+GET /lua
+--- error_log
+client prematurely closed connection
+retrieve_voms_ac_from_proxy
+plain http
+--- no_error_log
+signal 11
+--- error_code: 200