Fix arithmetic overflow in umul192_lower128.

Doesn't affect release builds, but extracting the mid 64 bits in
`umul192_lower128` leads to arithmetic overflow in `hi + (hi_lo >> 64)
as u64`, causing an undesirable panic.

Closes #84.

lexical-write-float/src/algorithm.rs

@@ -874,7 +874,8 @@ pub const fn umul192_upper128(x: u64, hi: u64, lo: u64) -> (u64, u64) {
 pub const fn umul192_lower128(x: u64, yhi: u64, ylo: u64) -> (u64, u64) {
     let hi = x.wrapping_mul(yhi);
     let hi_lo = x as u128 * ylo as u128;
-    (hi + (hi_lo >> 64) as u64, hi_lo as u64)
+    // NOTE: This can wrap exactly to 0, and this is desired.
+    (hi.wrapping_add((hi_lo >> 64) as u64), hi_lo as u64)
 }
 
 #[inline(always)]

lexical-write-float/tests/algorithm_tests.rs

@@ -91,6 +91,13 @@ fn floor_log10_pow2_minus_log10_4_over_3_test() {
     }
 }
 
+#[test]
+fn issue84_test() {
+    let (hi, lo) = algorithm::umul192_lower128(15966911296221875, 0xcccccccccccccccc, 0xcccccccccccccccd);
+    assert_eq!(hi, 0);
+    assert_eq!(lo, 3193382259244375);
+}
+
 #[test]
 fn pow32_test() {
     assert_eq!(algorithm::pow32(10, 1), 10);