Realip: fixed duplicate processing on redirects (ticket #1098).

Duplicate processing was possible if the address set by realip was
listed in set_realip_from, and there was an internal redirect so module
context was cleared.  This resulted in exactly the same address being set,
so this wasn't a problem before the $realip_remote_addr variable was
introduced, though now results in incorrect $realip_remote_addr being
picked.

Fix is to use ngx_http_realip_get_module_ctx() to look up module context
even if it was cleared.  Additionally, the order of checks was switched to
check the configuration first as it looks more effective.

src/http/modules/ngx_http_realip_module.c

diff --git a/src/http/modules/ngx_http_realip_module.c b/src/http/modules/ngx_http_realip_module.c
index dba3c52..5e3355c 100644
--- a/src/http/modules/ngx_http_realip_module.c
+++ b/src/http/modules/ngx_http_realip_module.c
@@ -141,18 +141,18 @@
     ngx_http_realip_ctx_t       *ctx;
     ngx_http_realip_loc_conf_t  *rlcf;
 
-    ctx = ngx_http_get_module_ctx(r, ngx_http_realip_module);
-
-    if (ctx) {
-        return NGX_DECLINED;
-    }
-
     rlcf = ngx_http_get_module_loc_conf(r, ngx_http_realip_module);
 
     if (rlcf->from == NULL) {
         return NGX_DECLINED;
     }
 
+    ctx = ngx_http_realip_get_module_ctx(r);
+
+    if (ctx) {
+        return NGX_DECLINED;
+    }
+
     switch (rlcf->type) {
 
     case NGX_HTTP_REALIP_XREALIP: