Core: guard against failed allocation during binary upgrade.

Patch by Piotr Sikora.

src/core/nginx.c

diff --git a/src/core/nginx.c b/src/core/nginx.c
index d02f911..e81e7af 100644
--- a/src/core/nginx.c
+++ b/src/core/nginx.c
@@ -594,6 +594,9 @@
     var = ngx_alloc(sizeof(NGINX_VAR)
                     + cycle->listening.nelts * (NGX_INT32_LEN + 1) + 2,
                     cycle->log);
+    if (var == NULL) {
+        return NGX_INVALID_PID;
+    }
 
     p = ngx_cpymem(var, NGINX_VAR "=", sizeof(NGINX_VAR));