don't let bin2hex call strlen

don't let bin2hex call strlen

Currently bin2hex may call strlen if the length argument is zero.
This prevents some function unification; and also it seems cleaner to
me not to have a special meaning for a zero length.

2014-02-12  Tom Tromey  <tromey@redhat.com>

	* common/rsp-low.c (bin2hex): Never take strlen of argument.
	* remote.c (extended_remote_run, remote_rcmd)
	(remote_download_trace_state_variable, remote_save_trace_data)
	(remote_set_trace_notes): Update.
	* tracepoint.c (encode_source_string, tfile_write_status)
	(tfile_write_uploaded_tsv): Update.