Return early in target_xfer_partial when LEN is zero.

Return early in target_xfer_partial when LEN is zero.

Nowadays, argument LEN of to_xfer_partial can be zero in some cases,
and each implementation may do nothing and return zero, indicating
transfer is done.  That is fine.  However, when we change
to_xfer_partial to return target_xfer_status, we have to check every
return value of most of to_xfer_partial implementations, return
TARGET_XFER_DONE if return value is zero.

This patch simplifies this by checking LEN in target_xfer_partial, and
return 0 if LEN is zero.  Regression tested on x86_84-linux.  Is it
OK?

gdb:

2014-02-07  Yao Qi  <yao@codesourcery.com>

	* target.c (target_xfer_partial): Return zero if LEN is zero.