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.
Hojun-Cho
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