Assertion `len == 1' failed, process aborted while streaming ouput from remote server

How it was found

Summary: During the course of testing putty-0.70-2019-02-12.75dda5e on Fedora 29 compiled with clang version 7.0.1 (Fedora 7.0.1-1.fc29), we discovered it was possible to abort a remote client by streaming data at it in such a way as to trigger an assertion failure.

putty: unix/gtkwin.c:3801: void do_text_internal(GtkFrontend *, int, int, wchar_t *, int, unsigned long, int, truecolour): Assertion `len == 1' failed.
Aborted (core dumped)

Description: An assertion is a statement that a predicate (Boolean-valued function, i.e. a true–false expression) is always true at that point in code execution. It can help a programmer read the code, help a compiler compile it, or help the program detect its own defects.

Steps To Reproduce:

1. Download PuTTY snapshot
2. Compile with Clang
3. Launch PuTTY with your favorite debugger.
4. Connection to remote host
5. On remote host: mkdir corpus && git clone https://gitlab.com/akihe/radamsa.git && cd radamsa && make && sudo make install && cd ~
6. On remote host, upload the attached files to the corpus directory we created in step 4.
7. On remote host type while true; radamsa -s 420 -o - -n inf corpus/*; done and let run until crashes.

Supporting Material/References:

A sample screenshot taken while fuzzing PuTTY: {F423359}

I've also attached the core dump that happened at the time of the crash.

Impact

Denial of service, crash, loss of data contained in scroll back