Crash clearing/filling a newly allocated ImageBuffer

[sentry-io]

Sentry Issue: ASEPRITE-1SH

EXCEPTION_ACCESS_VIOLATION_WRITE / 0x0: Fatal Error: EXCEPTION_ACCESS_VIOLATION_WRITE / 0x0
  File "memset.asm", line 35, in memset_repstos
  File "xutility", line 5045, in std::fill
    _Fill_memset(_UFirst, _Val, static_cast<size_t>(_ULast - _UFirst));
  File "image_impl.h", line 76, in doc::ImageImpl<T>::ImageImpl<T>
    std::fill(m_buffer->buffer(),
  File "image.cpp", line 54, in doc::Image::create
    case ColorMode::RGB:       return new ImageImpl<RgbTraits>(spec, buffer);
  File "image.cpp", line 42, in doc::Image::create
    return Image::create(ImageSpec((ColorMode)format, width, height, 0), buffer);
...
(45 additional frame(s) were not displayed)

[dacap]

We have to see if #4384 fixed this issue too (we marked the crash as resolved in Sentry, so if there is a regression we'll reopen this issue).