Sub filter: restored ngx_http_set_ctx() at the proper place.

Previously, ngx_http_sub_header_filter() could fail with a partially
initialized context, later accessed in ngx_http_sub_body_filter()
if called from the perl content handler.

The issue had appeared in 2c045e5b8291 (1.9.4).

A better fix would be to handle ngx_http_send_header() errors in
the perl module, though this doesn't seem to be easy enough.

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 de58c6f..6d3de59 100644
--- a/src/http/modules/ngx_http_sub_filter_module.c
+++ b/src/http/modules/ngx_http_sub_filter_module.c
@@ -248,8 +248,6 @@
                                  ctx->matches->nelts);
     }
 
-    ngx_http_set_ctx(r, ctx, ngx_http_sub_filter_module);
-
     ctx->saved.data = ngx_pnalloc(r->pool, ctx->tables->max_match_len - 1);
     if (ctx->saved.data == NULL) {
         return NGX_ERROR;
@@ -260,6 +258,8 @@
         return NGX_ERROR;
     }
 
+    ngx_http_set_ctx(r, ctx, ngx_http_sub_filter_module);
+
     ctx->offset = ctx->tables->min_match_len - 1;
     ctx->last_out = &ctx->out;