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/math/random/random.ww

@@ -28,6 +28,7 @@ export fn next(r: *random) u64 = {
 // fast unbiased mapping (mulhi-then-leftover-reject). Mirrors Hare's
 // random::u32n.
 export fn u32n(r: *random, n: u32) u32 = {
+	assert(n != 0u32);	// ref/hare/math/random/random.ha:26
 	let x: u32 = next(r): u32;
 	let prod: u64 = (x: u64) * (n: u64);
 	let leftover: u32 = prod: u32;
@@ -48,6 +49,7 @@ export fn u32n(r: *random, n: u32) u32 = {
 // path; otherwise rejection-sample to avoid modulo bias. Mirrors
 // Hare's random::u64n.
 export fn u64n(r: *random, n: u64) u64 = {
+	assert(n != 0u64);	// ref/hare/math/random/random.ha:42
 	if ((n & (n - 1u64)) == 0u64) { return next(r) & (n - 1u64); };
 	// max = U64_MAX - (U64_MAX+1) % n = -1 - (-n % n)
 	let neg: u64 = (0u64 - n);