bugfix: stream cosocket getsslctx() bound to wrong FFI symbol.

The stream branch in lib/resty/core/socket.lua bound
ngx_lua_ffi_socket_getsslctx to ngx_stream_lua_ffi_socket_tcp_get_ssl_pointer
(a copy/paste of the getsslpointer block above). As a result,
stream cosocket:getsslctx() returned an SSL* while callers used it as
SSL_CTX*, leading to undefined behavior. Bind to
ngx_stream_lua_ffi_socket_tcp_get_ssl_ctx instead, matching the http branch.

Author: perfgao

lib/resty/core/socket.lua

@@ -150,9 +150,9 @@ then
 ngx_lua_ffi_socket_getsslpointer =
     C.ngx_stream_lua_ffi_socket_tcp_get_ssl_pointer
 end
-if pcall(function() return C.ngx_stream_lua_ffi_socket_tcp_get_ssl_pointer end)
+if pcall(function() return C.ngx_stream_lua_ffi_socket_tcp_get_ssl_ctx end)
 then
-ngx_lua_ffi_socket_getsslctx = C.ngx_stream_lua_ffi_socket_tcp_get_ssl_pointer
+ngx_lua_ffi_socket_getsslctx = C.ngx_stream_lua_ffi_socket_tcp_get_ssl_ctx
 end
 
 if pcall(function()