Fix FALLBACK_SCSV parsing

Fixed a bug in ssl_srv.c when parsing TLS_FALLBACK_SCSV in the
ciphersuite list that caused it to miss it sometimes. Reported by Hugo
Leisink as issue #810. Fix initially by @andreasag01; this commit
isolates the bug fix and adds a non-regression test.
diff --git a/ChangeLog b/ChangeLog
index fe5ce65..47fec32 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.x.x branch released xxxx-xx-xx
+
+Security
+   * Fixed offset in FALLBACK_SCSV parsing that caused TLS server to fail to
+     detect it sometimes. Reported by Hugo Leisink. #810
+
 = mbed TLS 2.1.7 branch released 2017-03-08
 
 Security