fix(runtime-core) Fix a panic when generating globals.

Fix https://github.com/wasmerio/wasmer/issues/979.

When we try to get a global that doesn't exist, a panic is
generated. This patch just skip that path, and let a proper error be
generated later.

With this patch, we get:

```sh
$ cargo run -- run panic_index_oob_all_backends.wasm
Error: ExportNotFound { name: "main" }
```

which is kind of the expected behavior in such situation.

lib/runtime-core/src/backing.rs

@@ -450,6 +450,11 @@ impl LocalBacking {
             let value = match &global_init.init {
                 Initializer::Const(value) => value.clone(),
                 Initializer::GetGlobal(import_global_index) => {
+                    // Skip if the global hasn't been initialized properly.
+                    if imports.globals.len() <= import_global_index.index() {
+                        continue;
+                    }
+
                     imports.globals[*import_global_index].get()
                 }
             };