Fix an undersized buffer panic in multiplication

Reported-by: Guido Vranken <guidovranken@gmail.com>
Reported-by: Arvid Norberg <arvid.norberg@gmail.com>

src/biguint/multiplication.rs

@@ -174,7 +174,7 @@ fn mac3(mut acc: &mut [BigDigit], mut b: &[BigDigit], mut c: &[BigDigit]) {
 
         // 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();
+        let len = x1.len() + y1.len() + 1;
         let mut p = BigUint { data: vec![0; len] };
 
         // p2 = x1 * y1
@@ -350,7 +350,7 @@ fn mac3(mut acc: &mut [BigDigit], mut b: &[BigDigit], mut c: &[BigDigit]) {
 }
 
 fn mul3(x: &[BigDigit], y: &[BigDigit]) -> BigUint {
-    let len = x.len() + y.len();
+    let len = x.len() + y.len() + 1;
     let mut prod = BigUint { data: vec![0; len] };
 
     mac3(&mut prod.data, x, y);

tests/biguint.rs

@@ -1008,12 +1008,26 @@ fn test_checked_mul() {
 #[test]
 fn test_mul_overflow() {
     // Test for issue #187 - overflow due to mac3 incorrectly sizing temporary
-    let s = "531137992816767098689588206552468627329593117727031923199444138200403559860852242739162502232636710047537552105951370000796528760829212940754539968588340162273730474622005920097370111";
+    let s = "5311379928167670986895882065524686273295931177270319231994441382\
+             0040355986085224273916250223263671004753755210595137000079652876\
+             0829212940754539968588340162273730474622005920097370111";
     let a: BigUint = s.parse().unwrap();
     let b = a.clone();
     let _ = a.checked_mul(&b);
 }
 
+#[test]
+fn test_mul_overflow_2() {
+    // Try a bunch of sizes that are right on the edge of multiplication length
+    // overflow, where (x * x).data.len() == 2 * x.data.len() + 1.
+    for i in 1u8..20 {
+        let bits = 1u32 << i;
+        let x = (BigUint::one() << bits) - 1u32;
+        let x2 = (BigUint::one() << (2 * bits)) - &x - &x - 1u32;
+        assert_eq!(&x * &x, x2);
+    }
+}
+
 #[test]
 fn test_checked_div() {
     for elm in MUL_TRIPLES.iter() {