Fixing Retain to return MemoryHandle with correct PinnedPointer

src/mscorlib/shared/System/Buffers/MemoryHandle.cs

@@ -21,6 +21,18 @@ public unsafe struct MemoryHandle : IDisposable
             _handle = handle;
         }
 
+        internal void AddOffset(int offset)
+        {
+            if (_pointer == null)
+            {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.pointer);
+            }
+            else
+            {
+                _pointer = (void*)((byte*)_pointer + offset);
+            }
+        }
+
         [CLSCompliant(false)]
         public void* PinnedPointer => _pointer;
 

src/mscorlib/shared/System/Memory.cs

@@ -185,6 +185,7 @@ public unsafe MemoryHandle Retain(bool pin = false)
                 if (_index < 0)
                 {
                     memoryHandle = ((OwnedMemory<T>)_arrayOrOwnedMemory).Pin();
+                    memoryHandle.AddOffset((_index & RemoveOwnedFlagBitMask) * Unsafe.SizeOf<T>());
                 }
                 else
                 {

src/mscorlib/shared/System/ReadOnlyMemory.cs

@@ -170,6 +170,7 @@ public unsafe MemoryHandle Retain(bool pin = false)
                 if (_index < 0)
                 {
                     memoryHandle = ((OwnedMemory<T>)_arrayOrOwnedMemory).Pin();
+                    memoryHandle.AddOffset((_index & RemoveOwnedFlagBitMask) * Unsafe.SizeOf<T>());
                 }
                 else
                 {

src/mscorlib/src/System/ThrowHelper.cs

@@ -430,7 +430,8 @@ internal enum ExceptionArgument
         s,
         keyValuePair,
         input,
-        ownedMemory
+        ownedMemory,
+        pointer
     }
 
     //