Td4 is an array of u8. A u8 << int promotes the u8 to an int first then shifts.
If the mathematical result of a shift (as modelled by lhs * 2^{rhs}) is not
representable in an integer, behaviour is undefined. In other words, you can't
shift into the sign bit of a signed integer. Fix this by casting to u32
whenever we're shifting left by 24.

(For consistency, cast other shifts, too.)

Caught by -fsanitize=shift

Submitted by Nick Lewycky (Google)

(Imported from upstream's 8b37e5c14f0eddb10c7f91ef91004622d90ef361.)

Change-Id: Id0f98d1d65738533c6ddcc3c21bc38b569d74793
Reviewed-on: https://boringssl-review.googlesource.com/4040
Reviewed-by: Adam Langley <agl@google.com>