Port fix to crash in mixin checking

[ahejlsberg]

Ports microsoft/TypeScript#62928.

[Copilot]

Pull request overview

This PR ports a crash fix from TypeScript PR #62928 that prevents a nil pointer dereference in mixin type checking. The fix adds a guard condition in the getBaseTypes function to check if a type has the appropriate object flags before attempting to access its interface type data.

• Added guard condition to prevent crash when getBaseTypes is called on types without class/interface/reference object flags
• Added regression test case noCrashOnMixin2.ts that demonstrates the previously crashing mixin pattern
• Test properly produces type-checking errors without crashing the compiler

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/noCrashOnMixin2.ts	Adds regression test for mixin crash scenario with protected constructors
testdata/baselines/reference/compiler/noCrashOnMixin2.types	Expected type inference output for the test case
testdata/baselines/reference/compiler/noCrashOnMixin2.symbols	Expected symbol resolution output for the test case
testdata/baselines/reference/compiler/noCrashOnMixin2.errors.txt	Expected error messages showing proper error reporting without crash
internal/checker/checker.go	Adds guard condition in getBaseTypes to prevent nil pointer dereference on types without appropriate object flags