misc/wasm: make wasm_exec more robust against uncommon environments

JavaScript environments are quite unpredictable because bundlers add
mocks for compatibility and libraries can polute the global namespace.
Detect more of such situations:

- Add check that require("fs") returns an object.
- Fix check that require("fs") returns an non-empty object.
- Add check that "module" is defined.

Fixes #40730

Change-Id: I2ce65fc7db64bbbb0b60eec79a4cfe5c3fec99c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/248758
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

misc/wasm/wasm_exec.js

@@ -11,6 +11,7 @@
 	// - Node.js
 	// - Electron
 	// - Parcel
+	// - Webpack
 
 	if (typeof global !== "undefined") {
 		// global already exists
@@ -28,7 +29,7 @@
 
 	if (!global.fs && global.require) {
 		const fs = require("fs");
-		if (Object.keys(fs) !== 0) {
+		if (typeof fs === "object" && fs !== null && Object.keys(fs).length !== 0) {
 			global.fs = fs;
 		}
 	}
@@ -556,6 +557,7 @@
 	}
 
 	if (
+		typeof module !== "undefined" &&
 		global.require &&
 		global.require.main === module &&
 		global.process &&