Byte extract lowering of unions: convert width to bytes

The upper bound for lowering a union to a sequence of bytes is the
byte-width of the widest member, not the bit width.

src/util/lower_byte_operators.cpp

@@ -975,7 +975,7 @@ static exprt unpack_rec(
         member,
         little_endian,
         offset_bytes,
-        widest_member->second,
+        (widest_member->second + bits_per_byte - 1) / bits_per_byte,
         bits_per_byte,
         ns,
         true);