Out-of-bounds read/write and invalid free with `externref`s and GC safepoints in Wasmtime

Impact There was an invalid free and out-of-bounds read and write bug when running Wasm that uses externrefs in Wasmtime. To trigger this bug, Wasmtime needs to be running Wasm that uses externrefs, the host creates non-null externrefs, Wasmtime performs a garbage collection GC, and there has to ...