[BUG] Assertion failure on malformed type alias definition

[cpp-niel]

Describe the bug
The cppfront compiler terminates with an assertion failure when a type alias definition incorrectly uses = instead of ==.

To Reproduce
Steps to reproduce the behavior:

1. Sample code - distilled down to minimal essentials please
   The behaviour can be reproduced by trying to compile this code:

abc: type = {
    a: int;
}

xyz: type = abc;

2. Command lines including which C++ compiler you are using

simple compilation without any additional command line arguments triggers the problem

3. Expected result - what you expected to happen

A syntax error from the compiler

4. Actual result/error

cppfront: source/cppfront.cpp:5356: void cpp2::cppfront::emit(const cpp2::declaration_node&, const std::string&): Assertion `n.initializer && n.initializer->is_compound()' failed.
Program terminated with signal: SIGSEGV
Compiler returned: 139

Additional context
Compiler explorer link: https://godbolt.org/z/6ceYTeMYP

[hsutter]

Thanks!