Fix null pointer dereference in the RSA module.

Introduced null pointer checks in mbedtls_rsa_rsaes_pkcs1_v15_encrypt

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 1dba929..ea55df8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -17,6 +17,8 @@
    * Fix bug in mbedtls_x509_crt_parse that caused trailing extra data in the 
      buffer after DER certificates to be included in the raw representation.
    * Fix issue that caused a hang when generating RSA keys of odd bitlength
+   * Fix bug in mbedtls_rsa_rsaes_pkcs1_v15_encrypt that made null pointer
+     dereference possible.
 
 Changes
    * On ARM platforms, when compiling with -O0 with GCC, Clang or armcc5,