Fix potential buffer overrun in ClearWithReferences

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

Author: GrabYourPitchforks

src/System.Private.CoreLib/shared/System/SpanHelpers.cs

@@ -517,7 +517,6 @@ public static unsafe void ClearWithReferences(ref IntPtr ip, nuint pointerSizeLe
             // Write first four and last three.
             Unsafe.Add(ref ip, 2) = default(IntPtr);
             Unsafe.Add(ref ip, 3) = default(IntPtr);
-            Unsafe.Add(ref ip, 4) = default(IntPtr);
             Unsafe.Add(ref Unsafe.Add(ref ip, (IntPtr)pointerSizeLength), -3) = default(IntPtr);
             Unsafe.Add(ref Unsafe.Add(ref ip, (IntPtr)pointerSizeLength), -2) = default(IntPtr);