Make a check more explicit in aes.c
The check was already done later when calling ECB, (as evidenced by the tests
passing, which have a call with data_unit set to NULL), but it's more readable
to have it here too, and more helpful when debugging.
diff --git a/library/aes.c b/library/aes.c
index e48a2a6..f6dc996 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -1183,6 +1183,7 @@
AES_VALIDATE_RET( ctx != NULL );
AES_VALIDATE_RET( mode == MBEDTLS_AES_ENCRYPT ||
mode == MBEDTLS_AES_DECRYPT );
+ AES_VALIDATE_RET( data_unit != NULL );
AES_VALIDATE_RET( input != NULL );
AES_VALIDATE_RET( output != NULL );