fix $upstream_http_ variable prefix length

src/http/ngx_http_variables.c

diff --git a/src/http/ngx_http_variables.c b/src/http/ngx_http_variables.c
index 150c4e2..713df22 100644
--- a/src/http/ngx_http_variables.c
+++ b/src/http/ngx_http_variables.c
@@ -494,7 +494,7 @@
         return NULL;
     }
 
-    if (ngx_strncmp(name->data, "upstream_http_", 10) == 0) {
+    if (ngx_strncmp(name->data, "upstream_http_", 14) == 0) {
 
         if (ngx_http_upstream_header_variable(r, vv, (uintptr_t) name)
             == NGX_OK)