Make seek work on /dev/null

Seeking `/dev/null` should work and do nothing instead of returning `EPIPE`.
This fixes it.

Author: hoodmane

src/library_fs.js

@@ -1348,6 +1348,7 @@ FS.staticInit();
       FS.registerDevice(FS.makedev(1, 3), {
         read: () => 0,
         write: (stream, buffer, offset, length, pos) => length,
+        llseek: () => 0,
       });
       FS.mkdev('/dev/null', FS.makedev(1, 3));
       // setup /dev/tty and /dev/tty1

test/other/test_seek_dev_null.c

@@ -0,0 +1,21 @@
+#include <unistd.h>
+#include <fcntl.h>
+#include <string.h>
+#include <errno.h>
+#include <stdio.h>
+
+int main() {
+    int fd = open("/dev/null", O_WRONLY);
+    printf("fd: %d\n", fd);
+    if (fd == -1) {
+        printf("open failed: %s\n", strerror(errno));
+        return 1;
+    }
+    int res = lseek(fd, 10, SEEK_CUR);
+    printf("res: %d\n", res);
+    if (res == -1) {
+        printf("lseek failed: %s\n", strerror(errno));
+        return 1;
+    }
+    return 0;
+}

test/other/test_seek_dev_null.out

@@ -0,0 +1,2 @@
+fd: 3
+res: 0

test/test_other.py

@@ -15295,3 +15295,6 @@ def test_fp16(self, opts):
 
   def test_embool(self):
     self.do_other_test('test_embool.c')
+
+  def test_seek_dev_null(self):
+    self.do_other_test('test_seek_dev_null.c')