fix potential segfault,
found by Coverity's Scan

src/core/nginx.c

diff --git a/src/core/nginx.c b/src/core/nginx.c
index df6c72c..b758775 100644
--- a/src/core/nginx.c
+++ b/src/core/nginx.c
@@ -438,6 +438,9 @@
     }
 
     var = ngx_array_push(&ccf->env);
+    if (var == NULL) {
+        return NULL;
+    }
 
     var->len = 2;
     var->data = (u_char *) "TZ";