CompileHelper: guard null typespec in defaultPatternAssignment pattern op

[EylonKrause]

Problem

A struct/array assignment pattern whose member key is a cast expression segfaults during elaboration:

module top();
  typedef struct packed { int a; } s;
  s x = '{ int'(0): 1 };
endmodule

surelog -parse repro.sv → Segmentation fault.

Root cause

In CompileHelper::defaultPatternAssignment, the vpiAssignmentPatternOp case dereferences a possibly-null typespec:

const typespec* ptps = nullptr;
if (const UHDM::ref_typespec* rt = pattern->Typespec()) {
  ptps = rt->Actual_typespec();
}
if (ptps->VpiName() == "default") {   // segfault when ptps == nullptr

ptps stays null when the tagged pattern has no typespec. A '{key: value} member sets its typespec only when compileTypespec succeeds for the key; for a cast-expression key (int'(0), byte'(0), 4'(0)), compileTypespec returns null, so pattern->Typespec() is never set and ptps is null at the dereference.

The sibling vpiCastOp case a few lines above guards the identical Typespec() -> Actual_typespec() pattern with if (optps == nullptr) break;; the vpiAssignmentPatternOp case is missing it.

Fix

Add the same null guard before dereferencing ptps:

if (ptps == nullptr) break;

A cast-key pattern is then left unmodified instead of crashing.

Test

Not a *_test.cpp unit test (the crash is in a full-elaboration function); reproducible end-to-end with the 3-line module above (segfault before the fix, clean after).