Fix potential stack overflow in StdInReader (#68398)

* Fix potential stack overflow

* Reduce the size of the stack allocation

* Update src/libraries/System.Console/src/System/IO/StdInReader.cs

Co-authored-by: Theodore Tsirpanis <teo@tsirpanis.gr>

* Fix range check

Co-authored-by: Theodore Tsirpanis <teo@tsirpanis.gr>

src/libraries/System.Console/src/System/IO/StdInReader.cs

@@ -46,11 +46,15 @@ internal bool IsUnprocessedBufferEmpty()
 
         internal void AppendExtraBuffer(ReadOnlySpan<byte> buffer)
         {
-            // Ensure a reasonable upper bound applies to the stackalloc
-            Debug.Assert(buffer.Length <= 1024);
-
-            // Then convert the bytes to chars
-            Span<char> chars = stackalloc char[_encoding.GetMaxCharCount(buffer.Length)];
+            // Most inputs to this will have a buffer length of one.
+            // The cases where it is larger than one only occur in ReadKey
+            // when the input is not redirected, so those cases should be
+            // rare, so just allocate.
+            const int MaxStackAllocation = 256;
+            int maxCharsCount = _encoding.GetMaxCharCount(buffer.Length);
+            Span<char> chars = (uint)maxCharsCount <= MaxStackAllocation ?
+                stackalloc char[MaxStackAllocation] :
+                new char[maxCharsCount];
             int charLen = _encoding.GetChars(buffer, chars);
             chars = chars.Slice(0, charLen);