sha: fix strict aliasing warnings

sha256.c:492:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
sha256.c:784:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
sha256.c:785:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Mike Frysinger <vapier@gentoo.org>