Resolve xoshiro256** out of range exn

Fixed an OutOfRangeException thrown by the xoshiro256** implementation of DoSampleBytes when passed certain length buffers

Author: charlieturndorf

src/Numerics.Tests/Random/RandomTests.cs

@@ -137,6 +137,20 @@ public void ThreadSafeSample()
             t2.Join();
         }
 
+        /// <summary>
+        /// NextBytes fills buffers of different sizes without throwing an exception (e.g. OutOfRangeException)
+        /// </remarks>
+        [Test]
+        public void NextBytesDoesNotThrow()
+        {
+            var rng = (System.Random)Activator.CreateInstance(_randomType, new object[] { false });
+
+            for (int i = 1; i < 30; i++)
+            {
+                Assert.DoesNotThrow(() => rng.NextBytes(new byte[i]));
+            }
+        }
+
         /// <summary>
         /// Test runner function.
         /// </summary>

src/Numerics/Random/Xoshiro256StarStar.cs

@@ -158,7 +158,8 @@ protected override void DoSampleBytes(byte[] buffer)
             int i = 0;
 
             // Fill up the bulk of the buffer in chunks of 8 bytes at a time.
-            for (int bound = buffer.Length - 3; i < bound;)
+            int bound = buffer.Length - (buffer.Length % 8);
+            while (i < bound)
             {
                 // Generate 64 random bits.
                 ulong x = RotateLeft(s1 * 5, 7) * 9;