shell: fix index out bound issue

In order to prevent index out of bounds access of the sh->ctx->cmd_buff
array when sh->ctx->cmd_buff_pos is 0, it has been added a check for the
value of sh->ctx->cmd_buff_pos.

This ensures that the array will not be accessed beyond its boundaries.

Signed-off-by: Jungo Lin <jungolin.tw@gmail.com>