Fix a sixel crash when the buffer is reflowed (microsoft#17951)

## Summary of the Pull Request

The sixel parser has an internal buffer that holds the indexed-color
representation of the image, prior to it being translated to RGB. This
buffer only retains the section of the image that is within the visible
viewport, so we're continually erasing segments from the top of it when
the image is large enough to trigger a scroll.

But there is a problem that arises if the window or font is resized so
that the buffer needs to reflow, because that can result in the image
being pushed entirely offscreen. At that point the segment we're trying
to erase is actually larger than the buffer itself, which can end up
causing the terminal to crash

To fix this, we just need to check for an oversized erase attempt and
simply clear the buffer instead.

## Validation Steps Performed

I could easily reproduce this crash in Windows Terminal by resizing the
font while viewing an animated gif with img2sixel. With this PR applied
the crash no longer occurs.

## PR Checklist
- [x] Closes microsoft#17947

src/terminal/adapter/SixelParser.cpp

@@ -715,8 +715,16 @@ void SixelParser::_eraseImageBufferRows(const int rowCount, const til::CoordType
     const auto pixelCount = rowCount * _cellSize.height;
     const auto bufferOffset = rowOffset * _cellSize.height * _imageMaxWidth;
     const auto bufferOffsetEnd = bufferOffset + pixelCount * _imageMaxWidth;
-    _imageBuffer.erase(_imageBuffer.begin() + bufferOffset, _imageBuffer.begin() + bufferOffsetEnd);
-    _imageCursor.y -= pixelCount;
+    if (static_cast<size_t>(bufferOffsetEnd) >= _imageBuffer.size()) [[unlikely]]
+    {
+        _imageBuffer.clear();
+        _imageCursor.y = 0;
+    }
+    else
+    {
+        _imageBuffer.erase(_imageBuffer.begin() + bufferOffset, _imageBuffer.begin() + bufferOffsetEnd);
+        _imageCursor.y -= pixelCount;
+    }
 }
 
 void SixelParser::_maybeFlushImageBuffer(const bool endOfSequence)