Check return code of mbedtls_mpi_fill_random

Add MBEDTLS_MPI_CHK to check for error value of mbedtls_mpi_fill_random.
Reported and fix suggested by guidovranken in #740
diff --git a/ChangeLog b/ChangeLog
index 4181281..0bb5d88 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 MBEDTLS_MPI_CHK to check for error value of mbedtls_mpi_fill_random.
+     Reported and fix suggested by guidovranken in #740
 
 Features
    * Add the functions mbedtls_platform_setup() and mbedtls_platform_teardown()