[csharp] ByteString.CreateCodedInput should use ArraySegment offset a…

…nd count

CreateCodedInput is created from the underlying array behind the ByteString.
If this was created from a larger array (via Memory<byte> or ArrayPool etc)
then the CodedInput refers to the wrong section of memory.

Change is to add the offset and count like the other methods that use the
ArraySegment (ToString, ToBase64, WriteTo).

csharp/src/Google.Protobuf.Test/ByteStringTest.cs

@@ -210,6 +210,22 @@ public void UnsafeWrap()
             Assert.AreEqual(byte.MaxValue, s[0]);
         }
 
+        [Test]
+        public void CreateCodedInput_FromArraySegment()
+        {
+            byte[] data = new byte[] { 0, 1, 2, 3, 4, 5, 6 };
+            ByteString bs = UnsafeByteOperations.UnsafeWrap(data.AsMemory(2, 3));
+            CodedInputStream codedInputStream = bs.CreateCodedInput();
+
+            byte[] bytes = codedInputStream.ReadRawBytes(3);
+
+            Assert.AreEqual(3, bytes.Length);
+            Assert.AreEqual(2, bytes[0]);
+            Assert.AreEqual(3, bytes[1]);
+            Assert.AreEqual(4, bytes[2]);
+            Assert.IsTrue(codedInputStream.IsAtEnd);
+        }
+
         [Test]
         public void WriteToStream()
         {

csharp/src/Google.Protobuf/ByteString.cs

@@ -325,7 +325,7 @@ public CodedInputStream CreateCodedInput()
             if (MemoryMarshal.TryGetArray(bytes, out ArraySegment<byte> segment) && segment.Count == bytes.Length)
             {
                 // Fast path. ByteString was created with a complete array.
-                return new CodedInputStream(segment.Array);
+                return new CodedInputStream(segment.Array, segment.Offset, segment.Count);
             }
             else
             {