fix case 3 in tests/struct-types.idl

If I'm reading the source correctly, bit_offset is != NULL when the struct
will fit in a sub-word field. Because multiple such values can be packed in
one word, encode_packed_struct_inline()'s flush case should never occur since
the caller handles that already.

So wordval_fill > 0 condition was switched with bit_offset == NULL. This seems
to fix the test, and not break anything else (visibly, anyway).
note that the source is grim indeed

No documentation, no functional tests, no brakes, no helmet.