Add tests for overreads in pem_read_buffer()

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index ea7ce07..4cfcfeb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -4,7 +4,7 @@
 
 Bugfix
    * Fixed multiple buffer overreads in mbedtls_pem_read_buffer() when parsing
-     the input string in pem format to extract the different components. Found
+     the input string in PEM format to extract the different components. Found
      by Eyal Itkin.
 
 = mbed TLS 1.3.18 branch 2016-10-17