accumulator/forestproofs: fix off-by-one error in Prove sanity check

If the number of leaves is `numLeaves`, valid leaf positions
are in the range of `(0, numLeaves-1)`, i.e. `pos == numLeaves`
is already out of range. Take this into account for the sanity
check before calling `detectSubTreeRows`.

accumulator/forestproofs.go

@@ -25,7 +25,7 @@ func (f *Forest) Prove(wanted Hash) (Proof, error) {
 	}
 
 	// should never happen
-	if pos > f.numLeaves {
+	if pos >= f.numLeaves {
 		return pr, fmt.Errorf("prove: got leaf position %d but only %d leaves exist",
 			pos, f.numLeaves)
 	}