Add missing ret code checks in PEM module

Add missing return code checks in the functions pem_des_decrypt(),
pem_3des_decrypt() and pem_aes_decrypt() so that the calling function
mbedtls_pem_read_buffer() is notified of errors reported by the crypto
primitives AES, DES and 3DES.
diff --git a/ChangeLog b/ChangeLog
index 3e9bc2b..af7765c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,6 +3,12 @@
 = mbed TLS 2.1.x branch released xxxx-xx-xx
 
 Bugfix
+   * Fix unchecked return codes from AES, DES and 3DES functions in
+     pem_aes_decrypt(), pem_des_decrypt() and pem_des3_decrypt() respectively.
+     If a call to one of the functions of the cryptographic primitive modules
+     failed, the error may not be noticed by the function
+     mbedtls_pem_read_buffer() causing it to return invalid values. Found by
+     Guido Vranken. #756
    * Remove macros from compat-1.3.h that correspond to deleted items from most
      recent versions of the library. Found by Kyle Keen.
    * Fixed issue in mutexes to failing to initialise. #667