[library_sockfs.js] Cleanup buffer handling in sendmsg

Extracting the `buffer.slice` expression made the code cleanup and
showed the double `new Uint8Array` to be redundant (AFAICT).

src/library_sockfs.js

@@ -619,16 +619,13 @@ addToLibrary({
           buffer = buffer.buffer;
         }
 
-        var data;
-#if PTHREADS
-        // WebSockets .send() does not allow passing a SharedArrayBuffer, so clone the portion of the SharedArrayBuffer as a regular
-        // ArrayBuffer that we want to send.
-        if (buffer instanceof SharedArrayBuffer) {
-          data = new Uint8Array(new Uint8Array(buffer.slice(offset, offset + length))).buffer;
-        } else {
-#endif
-          data = buffer.slice(offset, offset + length);
+        var data = buffer.slice(offset, offset + length);
 #if PTHREADS
+        // WebSockets .send() does not allow passing a SharedArrayBuffer, so
+        // clone the the SharedArrayBuffer as regular ArrayBuffer before
+        // sending.
+        if (data instanceof SharedArrayBuffer) {
+          data = new Uint8Array(data).buffer;
         }
 #endif