Add bounds checks for offset in classes that impl OwnedMemory

Author: ahsonkhan

src/Common/tests/System/Buffers/NativeOwnedMemory.cs

@@ -55,8 +55,9 @@ protected override bool IsRetained
 
         public override unsafe MemoryHandle Pin(int offset = 0)
         {
+            if (offset < 0 || offset >= _length) throw new ArgumentOutOfRangeException(nameof(offset));
             void* pointer = (void*)((byte*)_ptr + offset);
-            return new MemoryHandle(this, _ptr);
+            return new MemoryHandle(this, pointer);
         }
 
         public override bool Release()

src/System.Memory/tests/Memory/CustomMemoryForTest.cs

@@ -39,13 +39,14 @@ public override Span<T> Span
             }
         }
 
-        public override MemoryHandle Pin(int index = 0)
+        public override MemoryHandle Pin(int offset = 0)
         {
             unsafe
             {
                 Retain();
+                if (offset < 0 || offset >= _array.Length) throw new ArgumentOutOfRangeException(nameof(offset));
                 var handle = GCHandle.Alloc(_array, GCHandleType.Pinned);
-                return new MemoryHandle(this, Unsafe.Add<byte>((void*)handle.AddrOfPinnedObject(), index), handle);
+                return new MemoryHandle(this, Unsafe.Add<byte>((void*)handle.AddrOfPinnedObject(), offset), handle);
             }
         }