fix(strutil): keep center-aligned Resize within the requested length

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.

strutil/padding.go

@@ -164,7 +164,9 @@ func resize(s string, sLen, wantLen int, align comdef.Align, cutOverflow bool) s
 
 	if align == comdef.Center {
 		padLn := (wantLen - sLen) / 2
-		if diff = wantLen - padLn*2; diff > 0 {
+		// add a single extra space only when the total padding is odd,
+		// otherwise the result overflows the requested length.
+		if diff = wantLen - sLen - padLn*2; diff > 0 {
 			s += " "
 		}
 		padStr := string(RepeatBytes(' ', padLn))