Merge branch 'josh/fix-write-bug' into 'master'

Fix a nasty bug in write(). DOn't allow writing to the keyboard!

See merge request !23

userprog/syscall_write.c

@@ -21,7 +21,10 @@ void syscall_write(struct intr_frame *f) {
     unsigned size = *((unsigned*)((int*)f->esp + 3)); // size to write is fourth
     // writing to stdout or stderr (fd in {1, 2}) is a special case
     switch (fd) {
-    case 1: // stdin
+    case 0: // writing to stdin (keyboard) is an error condition
+        f->eax = 0;
+        break;
+    case 1: // stdout
     case 2: // stderr
         write_to_console((const char*)buffer, size);
         f->eax = size;