Ensure BIGNUM is freed in an error path

Reviewed-by: Richard Levitte <levitte@openssl.org>
diff --git a/apps/prime.c b/apps/prime.c
index 940fd45..b0f5969 100644
--- a/apps/prime.c
+++ b/apps/prime.c
@@ -119,9 +119,8 @@
         }
     }
 
-    BN_free(bn);
-
     ret = 0;
  end:
+    BN_free(bn);
     return ret;
 }