In thunk_type_align() and thunk_type_size() we currently return
-1 if the value at the type_ptr isn't one of the TYPE_* values
we understand. However, this should never happen, and if it does
then the calling code will go confusingly wrong because none
of the callsites try to handle an error return. Switch to an
assertion instead, so that if this does somehow happen we'll have
a nice clear backtrace of what happened rather than a weird crash
or misbehaviour.

This also silences various Coverity complaints about not handling
the negative return value (CID 1005735, 1005736, 1005738, 1390582).
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NLaurent Vivier <laurent@vivier.eu>
Message-Id: <20180514174616.19601-1-peter.maydell@linaro.org>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>