Added syntax checking of the second parameter of the "split_clients" directive.

src/http/modules/ngx_http_split_clients_module.c

diff --git a/src/http/modules/ngx_http_split_clients_module.c b/src/http/modules/ngx_http_split_clients_module.c
index 726269c..33a2fe7 100644
--- a/src/http/modules/ngx_http_split_clients_module.c
+++ b/src/http/modules/ngx_http_split_clients_module.c
@@ -138,6 +138,13 @@
     }
 
     name = value[2];
+
+    if (name.len < 2 || name.data[0] != '$') {
+        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
+                           "invalid variable name \"%V\"", &name);
+        return NGX_CONF_ERROR;
+    }
+
     name.len--;
     name.data++;