Prevent mismatched checkout/checkin

Checkout can raise an exception if there is an error pulling connections
from the pool. In that case, we must not try to perform a checkin
because that will raise a new exception and mask the error caused by the
checkout.

lib/vault/persistent.rb

@@ -642,7 +642,10 @@ def connection_for uri
 
     raise Error, "host down: #{address}:#{port}"
   ensure
-    @pool.checkin net_http_args
+    # Only perform checkin if we successfully checked a connection out
+    if connection
+      @pool.checkin net_http_args
+    end
   end
 
   ##