fix: add null checks for getInterface() before getAllInterfaceFields()

[cs01]

Summary

• Add null checks after getInterface() in 3 methods in variable-allocator.ts before passing result to getAllInterfaceFields()
• Previously, if getInterface() returned null, the null was cast to InterfaceDeclaration and passed to getAllInterfaceFields(), causing a crash in the native compiler
• Fixed methods: allocateFunctionInterfaceReturn, allocateMethodInterfaceReturn, allocateMapGetInterface
• Now emits a clear compile-time error identifying the missing interface

Test plan

• All 437 tests pass
• Self-hosting verification passes (Stage 0 + Stage 1)

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