message: Add `WasmPtr` bug fix to changelog
diff: ## **[Unreleased]**
  +- [#1272](https://github.com/wasmerio/wasmer/pull/1272) Fix off-by-one error bug when accessing memory with a `WasmPtr` that contains the last valid byte of memory. Also changes the behavior of `WasmPtr<T, Array>` with a length of 0 and `WasmPtr<T>` where `std::mem::size_of::<T>()` is 0 to always return `None`
  +
  ## 0.15.0 - 2020-03-04
  - [#1263](https://github.com/wasmerio/wasmer/pull/1263) Changed the behavior of some WASI syscalls to now handle preopened directories more properly. Changed default `--debug` logging to only show Wasmer-related messages.
  