MM0 do_close() without IO was segfaulting. now fixed.

This was triggered by incorrect test0 build that omits to embed
the test executable.

tasks/mm0/src/file.c

@@ -528,6 +528,14 @@ int do_close(struct tcb *task, int fd)
 	if ((err = vfs_close(task->tid, fd)) < 0)
 		return err;
 
+	/*
+	 * If there was no IO on it, we may not know the file,
+	 * we simply return here. Since we notify VFS about the
+	 * close, it can tell us if the fd was open or not.
+	 */
+	if (!task->files->fd[fd].vmfile)
+		return 0;
+
 	/* Reduce file refcount etc. */
 	vm_file_put(task->files->fd[fd].vmfile);
 

tools/run-qemu-insight

@@ -1,4 +1,4 @@
-cd build
+cd build/loader
 qemu-system-arm -s -kernel final.axf -nographic -m 128 -M versatilepb &
 arm-none-insight ; pkill qemu-system-arm
 cd ..