cgen: default the hi bound of an N_DOT array-field slice arg

pushargsrev's N_SLICE arm resolved esz and the base address from the
stamped tinfo of an N_DOT [N]T-field base but let the omitted hi bound
fall to $0, so the #258 array-to-slice borrow desugar handed callees a
zero-length slice: base64 clear()'s bytes.zero(e.ibuf) was a runtime
no-op under the wwstage frontend. Take the element count from the
field's array tinfo like the cgslice #252 arm and cstage's bu->alen.
Graduates r989_libprecond_decodedsize_aligned and the 989_lib_byteid
#59.3 base64 pin.

test/wcc/989_lib_byteid.c

@@ -145,8 +145,10 @@ static const struct ent ents[] = {
 	{ .fixture = "lib/ascii/asciitest.ww",
 	  .mode = M_ID, .cite = "#59.1 graduated by #146 str==" },
 	/* #59.2 bufio graduated to M_ID above (#129 fix) */
+	/* #59.3 base64 graduated to M_ID (pushargsrev N_DOT array-field
+	 * default-hi fix — clear()'s borrow no longer zero-length) */
 	{ .fixture = "lib/encoding/base64/base64_test.ww",
-	  .mode = M_DIVERGE, .cite = "#59.3" },
+	  .mode = M_ID },
 	/* #59.4 hextest graduated to M_ID above (#22a reviewer fixes) */
 	{ .fixture = "lib/errors/errnotest.ww",
 	  .mode = M_DIVERGE, .cite = "#59.5" },