evaluate: check variadic argument types against format string

The variadic argument code did not check any of the variadic arguments
as it did not previously know the possible type. Now we have the possible
formatting information stored in the ctype, we can do some checks on the
printf formatting types.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>