Fix non-appends in WriteStream.write.

Author: whitequark

lib/wasi-virt.js

@@ -202,9 +202,10 @@ class WriteStream extends OutputStream {
   }
 
   write(contents) {
-    const newData = new Uint8Array(this.file.data.length + contents.length);
+    const offset = Number(this.offset);
+    const newData = new Uint8Array(Math.max(this.file.data.length, offset + contents.length));
     newData.set(this.file.data);
-    newData.subarray(Number(this.offset)).set(contents);
+    newData.subarray(offset).set(contents);
     this.file.data = newData;
     this.offset += BigInt(contents.length);
   }