Add ChangeLog entry for #375 fix

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rkuh4aSmrD8m2f2vYqakb6

Author: etr

ChangeLog

@@ -42,6 +42,9 @@ Version 0.20.0
 	Fixed std::terminate when MHD invokes the URI log callback with a
 		null uri pointer (e.g. port scans, half-open connections, or
 		non-HTTP traffic). Resolves issue #371.
+	Fixed std::terminate when MHD invokes the post-processor iterator with
+		a null key on a continuation chunk (a field split across multiple
+		callbacks). Resolves issue #375.
 
 Version 0.19.0 - 2023-06-15