Core: fixed resource leak if binary upgrade fails due to no memory.

Found by Coverity (CID 992320).
diff --git a/src/core/nginx.c b/src/core/nginx.c
index e81e7af..0459776 100644
--- a/src/core/nginx.c
+++ b/src/core/nginx.c
@@ -595,6 +595,7 @@
                     + cycle->listening.nelts * (NGX_INT32_LEN + 1) + 2,
                     cycle->log);
     if (var == NULL) {
+        ngx_free(env);
         return NGX_INVALID_PID;
     }