Contribution page with separate contribution and membership payments gives error if no payment processor

Use case: a contribution page is to be used for Pay Later only and is configured for separate contribution and membership payments. Hence no payment processor is selected, as credit card fields are not wanted. The following error occurs:

"This contribution page is configured to support separate contribution and membership payments. This plugin does not currently support multiple simultaneous payments. Please contact the site administrator and notify them of this error"

The attached patch against 3.3.5 resolves the problem by checking that there is a payment processor plugin before attempting to check its capabilities.