The center branch of resize() added the odd-padding space whenever the
input was non-empty, because the parity check omitted the source length
(wantLen - padLn*2 instead of wantLen - sLen - padLn*2). This made Resize,
Align and their utf8 variants overflow wantLen by one for every non-empty
string with an even amount of padding. Subtract sLen so the extra space is
only added when the total padding is actually odd.