fix segfault if there is single large_client_header_buffers
and a request line fills it completely

src/http/ngx_http_request.c

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index a0c4ea3..56eee26 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -954,9 +954,17 @@
                 }
 
                 if (rv == NGX_DECLINED) {
-                    len = r->header_in->end - r->header_name_start;
                     p = r->header_name_start;
 
+                    if (p == NULL) {
+                        ngx_log_error(NGX_LOG_INFO, c->log, 0,
+                                      "client sent too large request");
+                        ngx_http_finalize_request(r, NGX_HTTP_BAD_REQUEST);
+                        return;
+                    }
+
+                    len = r->header_in->end - p;
+
                     if (len > NGX_MAX_ERROR_STR - 300) {
                         len = NGX_MAX_ERROR_STR - 300;
                         p[len++] = '.'; p[len++] = '.'; p[len++] = '.';