Materialize non-null externrefs in the fuzzer (#4952)
Materialize non-null externrefs in the fuzzer (#4952)

Some fuzzer initial contents contain non-nullable externrefs that cause the
fuzzer to try to materialize non-nullable externref values. Perviously the
fuzzer did not support this and crashed with an assertion failure. Fix the
assertion failure by instead returning a null cast to non-null, which will trap
at runtime but at least produce a valid module.
WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen