Replace assertions with proper error handling in emitter and verifier

[byeongjee]

Problem

Several places use assert() for error conditions that can occur at runtime:

1. WasmStackEmitter.cpp:106 — assert(!emitter.labelStack.empty() && ...)
2. VerifyWasmStack.cpp:312 — assert(ctx && "MLIRContext not initialized")

Why this matters

assert() is stripped in release builds (-DNDEBUG). If these conditions are hit in a release build:

• Case 1: Proceeds with invalid state, causing undefined behavior
• Case 2: Null pointer dereference

Expected behavior

Replace assert() with proper error handling using emitError() / return failure() so that invalid states are caught regardless of build configuration.

Location

• lib/wasmstack/WasmStackEmitter.cpp, line 106
• lib/wasmstack/VerifyWasmStack.cpp, line 312