bigint: Fix calculation of size for multiply temporaries

When x.len() and y.len() are both equal and odd, we have x1.len() + y1.len() + 1
(the required size to multiply x1 and y1) > y.len() + 1

This fixes rust-num#187

bigint/src/lib.rs

@@ -991,9 +991,11 @@ fn mac3(acc: &mut [BigDigit], b: &[BigDigit], c: &[BigDigit]) {
         let (x0, x1) = x.split_at(b);
         let (y0, y1) = y.split_at(b);
 
-        /* We reuse the same BigUint for all the intermediate multiplies: */
-
-        let len = y.len() + 1;
+        /*
+         * We reuse the same BigUint for all the intermediate multiplies and have to size p
+         * appropriately here: x1.len() >= x0.len and y1.len() >= y0.len():
+         */
+        let len = x1.len() + y1.len() + 1;
         let mut p = BigUint { data: vec![0; len] };
 
         // p2 = x1 * y1