Added tests 616 and 617 to see how SFTP and SCP cope with zero-length

files, as questioned by Mike Protts. SFTP does for me but SCP doesn't
so test 617 is disabled for now.

tests/data/test617

@@ -0,0 +1,39 @@
+<testcase>
+<info>
+<keywords>
+SCP
+zero-length
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data sendzero="yes">
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+scp
+</server>
+ <name>
+SCP retrieval of empty file
+ </name>
+ <command>
+--key curl_client_key --pubkey curl_client_key.pub -u %USER: scp://%HOSTIP:%SSHPORT%PWD/log/file617.txt
+</command>
+<file name="log/file617.txt">
+</file>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<valgrind>
+disable
+</valgrind>
+</verify>
+</testcase>