Fix 'xfered>0' assertion in target.c for remote connection

Fix 'xfered>0' assertion in target.c for remote connection

We have a customer who is using a Corelis gdb server to connect to gdb.
Occasionally, the gdb server will send a 0-byte block of memory for a
read.  When this happens, gdb gives an assertion from target.c:

internal-error: target_xfer_partial: Assertion `*xfered_len > 0' failed.

This problem is almost identical to that fixed in
https://sourceware.org/ml/gdb-patches/2014-02/msg00636.html

In this case, remote.c needs to be modified to return TARGET_XFER_EOF
instead of TARGET_XFER_OK or TARGET_XFER_UNAVAILABLE when 0 bytes are
transferred.

gdb/ChangeLog:

	PR gdb/22388
	* remote.c (remote_write_bytes_aux, remote_read_bytes_1,
	remote_read_bytes, remote_write_qxfer, remote_xfer_partial):
	Return TARGET_XFER_EOF if size of returned data is 0.