extmod/vfs_fat_file: Make file.close() a no-op if file already closed.

As per CPython semantics.  In particular, file.__del__() should not raise
an exception if the file is already closed.

extmod/vfs_fat_file.c

@@ -120,9 +120,12 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_1(file_obj_flush_obj, file_obj_flush);
 
 STATIC mp_obj_t file_obj_close(mp_obj_t self_in) {
     pyb_file_obj_t *self = MP_OBJ_TO_PTR(self_in);
-    FRESULT res = f_close(&self->fp);
-    if (res != FR_OK) {
-        mp_raise_OSError(fresult_to_errno_table[res]);
+    // if fs==NULL then the file is closed and in that case this method is a no-op
+    if (self->fp.fs != NULL) {
+        FRESULT res = f_close(&self->fp);
+        if (res != FR_OK) {
+            mp_raise_OSError(fresult_to_errno_table[res]);
+        }
     }
     return mp_const_none;
 }

tests/extmod/vfs_fat_fileio.py

@@ -48,6 +48,7 @@ print(str(f)[:17], str(f)[-1:])
 f.write("hello!")
 f.flush()
 f.close()
+f.close() # allowed
 try:
     f.write("world!")
 except OSError as e:
@@ -63,11 +64,6 @@ try:
 except OSError as e:
     print(e.args[0] == uerrno.EINVAL)
 
-try:
-    f.close()
-except OSError as e:
-    print(e.args[0] == uerrno.EINVAL)
-
 try:
     vfs.open("foo_file.txt", "x")
 except OSError as e:

tests/extmod/vfs_fat_fileio.py.exp

@@ -3,7 +3,6 @@ True
 True
 True
 True
-True
 hello!world!
 12
 h