Backport 1.3: Check rc of mbedtls_mpi_fill_random

Add MPI_CHK to check for error value of mpi_fill_random.
Backport from Report and fix suggestion by guidovranken in #740
diff --git a/ChangeLog b/ChangeLog
index 43ad0c3..081bcf1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -11,6 +11,8 @@
    * Fix resource leak on windows platform, in mbedtls_x509_crt_parse_path.
      In case of failure, when an error occures, goto cleanup.
      Found by redplait #590
+   * Add MPI_CHK to check for error value of mpi_fill_random.
+     Backported from a report and fix suggestion by guidovranken in #740
 
 = mbed TLS 1.3.20 branch released 2017-06-21