The docs also say that the first expr_list can be null:
If @var{lval} is a @code{parallel}, it is used to represent the case of
a function returning a structure in multiple registers. Each element
of the @code{parallel} is an @code{expr_list} whose first operand is a
@code{reg} and whose second operand is a @code{const_int} representing the
offset (in bytes) into the structure at which the data in that register
corresponds. The first element may be null to indicate that the structure
is also passed partly in memory.
but I can't see any code to handle that. Am I missing something,
or does the lack of a crash here mean that we can remove the last
sentence?
(It might have been added for symmetry with argument passing, where this
sort of thing is needed. But if it isn't actually used or implemented for
returns, it might be less confusing to remove it.)