fix add_before_body without add_after_body

src/http/modules/ngx_http_addition_filter_module.c

diff --git a/src/http/modules/ngx_http_addition_filter_module.c b/src/http/modules/ngx_http_addition_filter_module.c
index 2651714..c05aefd 100644
--- a/src/http/modules/ngx_http_addition_filter_module.c
+++ b/src/http/modules/ngx_http_addition_filter_module.c
@@ -152,6 +152,11 @@
         }
     }
 
+    if (conf->after_body.len == 0) {
+        ngx_http_set_ctx(r, NULL, ngx_http_addition_filter_module);
+        return ngx_http_next_body_filter(r, in);
+    }
+
     last = 0;
 
     for (cl = in; cl; cl = cl->next) {