Avoid unnecessary sign-extension

Author: xtqqczze

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/SafeBuffer.cs

@@ -246,7 +246,7 @@ public void ReadSpan<T>(ulong byteOffset, Span<T> buffer)
                 ref T structure = ref MemoryMarshal.GetReference(buffer);
                 for (int i = 0; i < buffer.Length; i++)
                 {
-                    Unsafe.Add(ref structure, i) = Unsafe.ReadUnaligned<T>(ptr + alignedSizeofT * i);
+                    Unsafe.Add(ref structure, (nint)(uint)i) = Unsafe.ReadUnaligned<T>(ptr + (nuint)alignedSizeofT * (uint)i);
                 }
             }
             finally
@@ -335,7 +335,7 @@ public void WriteSpan<T>(ulong byteOffset, ReadOnlySpan<T> data)
                 ref T structure = ref MemoryMarshal.GetReference(data);
                 for (int i = 0; i < data.Length; i++)
                 {
-                    Unsafe.WriteUnaligned(ptr + alignedSizeofT * i, Unsafe.Add(ref structure, i));
+                    Unsafe.WriteUnaligned(ptr + (nuint)alignedSizeofT * (uint)i, Unsafe.Add(ref structure, (nint)(uint)i));
                 }
             }
             finally