Explicitly allow NULL as an argument to mbedtls_ccm_free()
diff --git a/library/ccm.c b/library/ccm.c
index 4bb3642..ad0d71b 100644
--- a/library/ccm.c
+++ b/library/ccm.c
@@ -106,7 +106,8 @@
  */
 void mbedtls_ccm_free( mbedtls_ccm_context *ctx )
 {
-    CCM_VALIDATE( ctx != NULL );
+    if( ctx == NULL )
+        return;
     mbedtls_cipher_free( &ctx->cipher_ctx );
     mbedtls_platform_zeroize( ctx, sizeof( mbedtls_ccm_context ) );
 }