Proxy: clear script engine used to calculate lengths.

Previous code is believed to be safe, but might access uninitialized
memory (e.g., e->quote).
diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index 88e41e0..d5cf07d 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -993,6 +993,8 @@
 
     len += uri_len;
 
+    ngx_memzero(&le, sizeof(ngx_http_script_engine_t));
+
     ngx_http_script_flush_no_cacheable_variables(r, plcf->flushes);
 
     if (plcf->body_set_len) {