Upstream: ngx_http_upstream_store() error handling fixes.

Previously, ngx_http_map_uri_to_path() errors were not checked in
ngx_http_upstream_store().  Moreover, in case of errors temporary
files were not deleted, as u->store was set to 0, preventing cleanup
code in ngx_http_upstream_finalize_request() from removing them.  With
this patch, u->store is set to 0 only if there were no errors.

Reported by Feng Gu.
diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index b8812cf..58edc62 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -3253,7 +3253,6 @@
                         || u->headers_in.content_length_n == tf->offset))
                 {
                     ngx_http_upstream_store(r, u);
-                    u->store = 0;
                 }
             }
         }
@@ -3375,7 +3374,9 @@
 
     if (u->conf->store_lengths == NULL) {
 
-        ngx_http_map_uri_to_path(r, &path, &root, 0);
+        if (ngx_http_map_uri_to_path(r, &path, &root, 0) == NULL) {
+            return;
+        }
 
     } else {
         if (ngx_http_script_run(r, &path, u->conf->store_lengths->elts, 0,
@@ -3393,6 +3394,8 @@
                    tf->file.name.data, path.data);
 
     (void) ngx_ext_rename_file(&tf->file.name, &path, &ext);
+
+    u->store = 0;
 }