Geo: ensure that default entry is always present.

If 0.0.0.0/32 entry was present and there was no explicit "default",
we failed to add an empty string as a default value.
diff --git a/src/http/modules/ngx_http_geo_module.c b/src/http/modules/ngx_http_geo_module.c
index 24091bf..26f4e57 100644
--- a/src/http/modules/ngx_http_geo_module.c
+++ b/src/http/modules/ngx_http_geo_module.c
@@ -453,16 +453,14 @@
         ngx_destroy_pool(ctx.temp_pool);
         ngx_destroy_pool(pool);
 
-        if (ngx_radix32tree_find(ctx.tree, 0) != NGX_RADIX_NO_VALUE) {
-            return rv;
-        }
-
         if (ngx_radix32tree_insert(ctx.tree, 0, 0,
                                    (uintptr_t) &ngx_http_variable_null_value)
             == NGX_ERROR)
         {
             return NGX_CONF_ERROR;
         }
+
+        /* NGX_BUSY is okay (default was set explicitly) */
     }
 
     return rv;