Variables: fixed buffer over-read when evaluating "$arg_".

src/http/ngx_http_variables.c

diff --git a/src/http/ngx_http_variables.c b/src/http/ngx_http_variables.c
index e067cf0..c2113c8 100644
--- a/src/http/ngx_http_variables.c
+++ b/src/http/ngx_http_variables.c
@@ -1075,7 +1075,7 @@
     len = name->len - (sizeof("arg_") - 1);
     arg = name->data + sizeof("arg_") - 1;
 
-    if (ngx_http_arg(r, arg, len, &value) != NGX_OK) {
+    if (len == 0 || ngx_http_arg(r, arg, len, &value) != NGX_OK) {
         v->not_found = 1;
         return NGX_OK;
     }