[BUG] Assertion `std::ssize(preprocessor) > 1' failed.

[MarekKnapek]

Steps to reproduce:

• Get latest cppfront source code.
• Compile cppfront, g++ -std=c++20 -o cppfront cppfront.cpp.
• Run cppfront on this test file:

#else#Xt:=private:E##)$##$

Expected result:

• Compilation success, or nice error message printed.

Actual result:

• cppfront: io.h:471: void cpp2::braces_tracker::found_pre_else(): Assertion `std::ssize(preprocessor) > 1' failed.
• Aborted (core dumped)

Found by fuzzing, #1131.

[hsutter]

Thanks! I've also edited this one to paste the source code directly into the issue, to avoid clicking on links to zipfiles (malware potential).