Fix double BIO_free in req

Reviewed-by: Tim Hudson <tjh@openssl.org>
diff --git a/apps/req.c b/apps/req.c
index a0e0cc9..a4cf312 100644
--- a/apps/req.c
+++ b/apps/req.c
@@ -591,6 +591,7 @@
             goto end;
         }
         BIO_free(out);
+        out = NULL;
         BIO_printf(bio_err, "-----\n");
     }