tests: fix stream get_upstream_ssl_pointer test for the log phase.

TEST 31 in t/stream/ssl.t relied on the previous 'not ssl' bug in
ngx.ssl.get_upstream_ssl_pointer(), which never detected a NULL cdata
and so masked that the upstream connection is already released by the
time log_by_lua runs in the stream subsystem. With the NULL check fixed
the API now correctly returns 'not upstream' in that phase.

Update the test to assert the real behaviour (and guard against the
NULL-detection regression) instead of expecting a stale 'session reused:
false' from ssl_session_reused() on a NULL pointer.

Author: zhuizhuhaomeng

t/stream/ssl.t

@@ -2517,9 +2517,11 @@ qr/ssl pointer: cdata<void \*>: 0x[0-9a-f]+,/
 
 
 
-=== TEST 31: get upstream SSL pointer
+=== TEST 31: get upstream SSL pointer (released in the log phase)
 Just for test api.
-The ssl_session_reuesed is always false in log_by_lua phase.
+In the stream subsystem the upstream connection is already released by the
+time log_by_lua runs, so get_upstream_ssl_pointer() reports "not upstream"
+instead of returning a stale/NULL pointer.
 --- stream_config
     lua_package_path "$TEST_NGINX_LUA_PACKAGE_PATH";
 
@@ -2537,17 +2539,17 @@ The ssl_session_reuesed is always false in log_by_lua phase.
     log_by_lua_block {
         local ssl = require "ngx.ssl"
         local ssl_conn, err = ssl.get_upstream_ssl_pointer()
-        if err ~= nil then
-            ngx.log(ngx.ERR, "failed to get ssl pointer: ", err)
+        if not ssl_conn then
+            ngx.log(ngx.WARN, "failed to get upstream ssl pointer: ", err)
             return
         end
         ngx.log(ngx.INFO, "session reused: ", ssl.ssl_session_reused(ssl_conn))
     }
 
 --- stream_response
 it works!
---- error_log eval
-qr/session reused: false/
+--- error_log
+failed to get upstream ssl pointer: not upstream
 --- no_error_log
 [alert]
 [crit]