Request body: added alert to catch duplicate body saving.

If due to an error ngx_http_request_body_save_filter() is called
more than once with rb->rest == 0, this used to result in a segmentation
fault.  Added an alert to catch such errors, just in case.

src/http/ngx_http_request_body.c

diff --git a/src/http/ngx_http_request_body.c b/src/http/ngx_http_request_body.c
index b72614f..c5401fd 100644
--- a/src/http/ngx_http_request_body.c
+++ b/src/http/ngx_http_request_body.c
@@ -1246,6 +1246,12 @@
 
     if (rb->temp_file || r->request_body_in_file_only) {
 
+        if (rb->bufs && rb->bufs->buf->in_file) {
+            ngx_log_error(NGX_LOG_ALERT, r->connection->log, 0,
+                          "body already in file");
+            return NGX_HTTP_INTERNAL_SERVER_ERROR;
+        }
+
         if (ngx_http_write_request_body(r) != NGX_OK) {
             return NGX_HTTP_INTERNAL_SERVER_ERROR;
         }