Fix undefined shifts

- in x509_profile_check_pk_alg
- in x509_profile_check_md_alg
- in x509_profile_check_key

and in ssl_cli.c : unsigned char gets promoted to signed integer
diff --git a/ChangeLog b/ChangeLog
index e2c68a9..35324cf 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.7.6 branch released xxxx-xx-xx
+
+Bugfix
+   * Fix undefined shifts with negative values in certificates parsing
+     (found by Catena cyber using oss-fuzz)
+
 = mbed TLS 2.7.5 branch released 2018-07-25
 
 Security