fs: fat: check for buffer size before reading blocks

This patch optimizes the commit mentioned below by avoiding running
a set of commands which are useless in the case when
size < mydata->sect_size and sect_count would be 0.

Fixes: 5b3ddb17 ("fs/fat/fat.c: Do not perform zero block reads if there are no blocks left")

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Co-developed-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>