Using correct pointer and size for memzero operation.

Found by Coverity (CID 215689).

src/nxt_h1proto.c

@@ -862,9 +862,8 @@ nxt_h1p_keepalive(nxt_task_t *task, nxt_h1proto_t *h1p, nxt_conn_t *c)
 
     b = h1p->buffers;
 
-    nxt_memzero(&h1p->parser, sizeof(nxt_h1proto_t));
+    nxt_memzero(h1p, offsetof(nxt_h1proto_t, conn));
 
-    h1p->conn = c;
     in = c->read;
 
     size = nxt_buf_mem_used_size(&in->mem);

src/nxt_http.h

@@ -55,8 +55,12 @@ typedef struct {
     nxt_http_te_t                   transfer_encoding:8;  /* 2 bits */
 
     nxt_http_request_t              *request;
-    nxt_conn_t                      *conn;
     nxt_buf_t                       *buffers;
+    /*
+     * All fields before the conn field will
+     * be zeroed in a keep-alive connection.
+     */
+    nxt_conn_t                      *conn;
 } nxt_h1proto_t;