Fix bug where not failing if out of memory

Still failing multi-oom.
Also fails syn-read and exec-missing on bochs.

src/threads/thread.c

@@ -604,6 +604,10 @@ int next_fd(struct thread *cur) {
 int add_open_file(struct thread *cur, struct file *file, int fd) {
     /* Initialize file */
     struct sys_file *new_file = palloc_get_page(PAL_ZERO);
+    /* Not enough memory. */
+    if (new_file == NULL) {
+        return -1;
+    }
     memset(new_file, 0, sizeof *new_file);
     new_file->file = file;
     new_file->fd = fd;

src/userprog/syscall.c

@@ -267,7 +267,11 @@ int sys_open(const char *file) {
     fd = add_open_file(cur, open_file, fd);
     release_file_lock();
 
-    ASSERT(fd > 1); /* Only for stdin and stdout */
+    if (fd == -1) {
+        acquire_file_lock();
+        file_close(open_file);
+        release_file_lock();
+    }
     return fd;
 }