opus

Fix an MSVC warning.

Microsoft Visual Studio 2010 warns about 'C4146: unary minus
operator applied to unsigned type, result still unsigned'
because of the '&-sizeof(void*)' in align().

This commit works around the warning by casting the size_t
to int before negation.

Patch by Hauke, who reported the issue on the opus mailing
list. Reviewed by derf.