misc/wasm: fix command line arguments containing multi-byte characters

Command line arguments containing multi-byte characters were causing
go_js_wasm_exec to crash (RangeError: Source is too large), because
their byte length was not handled correctly. This change fixes the bug.

Fixes #31645.

Change-Id: I7860ebf5b12da37d9d0f43d4b6a22d326a90edaf
Reviewed-on: https://go-review.googlesource.com/c/go/+/173877
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/wasm/wasm_exec.js

@@ -415,9 +415,13 @@
 			let offset = 4096;
 
 			const strPtr = (str) => {
-				let ptr = offset;
-				new Uint8Array(mem.buffer, offset, str.length + 1).set(encoder.encode(str + "\0"));
-				offset += str.length + (8 - (str.length % 8));
+				const ptr = offset;
+				const bytes = encoder.encode(str + "\0");
+				new Uint8Array(mem.buffer, offset, bytes.length).set(bytes);
+				offset += bytes.length;
+				if (offset % 8 !== 0) {
+					offset += 8 - (offset % 8);
+				}
 				return ptr;
 			};