sixel: prevent allocating an absurd amount of memory or writing OOB (#20213)

This commit implements two fixes for the integer overflow/out-of-bounds
write reported in #20149.

First, it catches any exception generated in sixel char processing
(which will prevent `out_of_memory` or `bad_alloc` from being ignored
sight-unseen, and will prevent the consumption of further DCS content).

Second, it prevents us from allocating memory for an image which will
never be displayed (because it exceeds the height of the display.)

This supersedes prior work in #20153 for the same issues.

Closes #20149
Closes #20153

Co-authored-by: James Holderness <j4_james@hotmail.com>

src/terminal/adapter/SixelParser.cpp

@@ -90,7 +90,15 @@ std::function<bool(wchar_t)> SixelParser::DefineImage(const VTInt macroParameter
         _state = States::Normal;
         _parameters.clear();
         return [&](const auto ch) {
-            _parseCommandChar(ch);
+            try
+            {
+                _parseCommandChar(ch);
+            }
+            catch (...)
+            {
+                // Ignore all further content.
+                return false;
+            }
             return true;
         };
     }
@@ -234,10 +242,18 @@ void SixelParser::_executeNextLine()
     _executeCarriageReturn();
     _imageLineCount++;
     _maybeFlushImageBuffer();
-    _imageCursor.y += _sixelHeight;
-    _availablePixelHeight -= _sixelHeight;
-    _resizeImageBuffer(_sixelHeight);
-    _fillImageBackgroundWhenScrolled();
+    // If we don't have any available pixel height, that means the image has
+    // extended beyond the bottom of the display and we haven't triggered a
+    // a scroll (because sixel display mode is enabled). In this state, there
+    // is no point in extending the image any further, because the additional
+    // content will never be seen, so we'll just be wasting memory.
+    if (_availablePixelHeight > 0)
+    {
+        _imageCursor.y += _sixelHeight;
+        _availablePixelHeight -= _sixelHeight;
+        _resizeImageBuffer(_sixelHeight);
+        _fillImageBackgroundWhenScrolled();
+    }
 }
 
 void SixelParser::_executeMoveToHome()