Sub filter: fixed initialization in http{} level (ticket #791).

If sub_filter directive was only specified at http{} level, sub filter
internal data remained uninitialized.  That would lead to a crash in runtime.

src/http/modules/ngx_http_sub_filter_module.c

diff --git a/src/http/modules/ngx_http_sub_filter_module.c b/src/http/modules/ngx_http_sub_filter_module.c
index 0a4ff6d..8ba75ed 100644
--- a/src/http/modules/ngx_http_sub_filter_module.c
+++ b/src/http/modules/ngx_http_sub_filter_module.c
@@ -853,8 +853,9 @@
         conf->pairs = prev->pairs;
         conf->matches = prev->matches;
         conf->tables = prev->tables;
+    }
 
-    } else if (conf->dynamic == 0){
+    if (conf->pairs && conf->dynamic == 0 && conf->tables == NULL) {
         pairs = conf->pairs->elts;
         n = conf->pairs->nelts;