lib/math+encoding: restore Hare loud preconditions (F-Q)

Three lib functions had lost their Hare loud-abort preconditions, so an
out-of-domain argument silently returned garbage instead of aborting:

  random.u32n / random.u64n  assert(n != 0)    ref/hare/math/random/random.ha:26,42
  base64.decodedsize         assert(sz%4 == 0) ref/hare/encoding/base64/base64.ha:597

Source-bundled lib change, identical on both stages (byte-id neutral).
989_libprecond_abort pins each precondition: n=0 / sz%4!=0 abort (rc!=0),
valid args return 0, run on cstage and wwstage.

lib/encoding/base64/base64.ww

@@ -413,5 +413,6 @@ export fn encodedsize(sz: i32) i32 = {
 // length is up to 2 bytes shorter, depending on padding). `sz` must be a
 // multiple of 4. ref/hare/encoding/base64/base64.ha:596-599.
 export fn decodedsize(sz: i32) i32 = {
+	assert(sz % 4i32 == 0i32);	// ref/hare/encoding/base64/base64.ha:597
 	return sz / 4 * 3;
 };