Can't create Credit Memo from Customer RMA when original invoice is multishipment

Having triaged it myself I now ran into the problem with a customer of mine.

Problem appears as follows:

MRMA.getOriginalInvoice fails.
first it tries to get the invoice from m_inout which is connected to rma m_inout. But no m_inout has a c_invoice_id other than null.
second it tries to get the invoice from c_order, but the invoice, where this order was invoiced was an invoice for two orders and is connected to another order. So this fails, too.

Because we allow to invoice more than one order with one invoice, connection can be done only on line level and of course it must be a line specific to this rma/shipment.

triaged by @Martin Schönbeck as not reproducible in 7.1z

with two orders, two shipments and one invoice rma could be generated on items of the first or second shipment, as well as for the full position as for a partial position.

Clearification:
Multiple orders, multiple shipments goes into one single invoice.
The problem description probably should have been
".... when original invoice is multiorder", since I think it was the multiple order that was the problem, not the multiple shipment which is a side effect of multiple orders.

All invoices are generated from orders. No customization on this part.

@Daniel Tamm, I was not able to reproduce the issue - seems like you're generating invoices not associated to an order? Is that a customization?

See https://bitbucket.org/dantam/idempiere.se/commits/1021200bb0be270227dd554e2829273756e6940e

for code change suggestion.