crypto/ripemd160: Correct badly sized union member

crypto/ripemd160: Correct badly sized union member

struct ripemd160_ctx has a union for converting between u8[] and u32[]
data.  Unfortunately the u32 array has a miscalculated size, half the size
of the u8 array.  That means some accesses which are within the union can
technically overrun the u32 array.

Found by Coverity scan.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>