Use ROUND_TO_ALIGN in scratch_create

Author: jonasnick

src/scratch_impl.h

@@ -11,7 +11,7 @@
 #include "scratch.h"
 
 static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t size) {
-    const size_t base_alloc = ((sizeof(secp256k1_scratch) + ALIGNMENT - 1) / ALIGNMENT) * ALIGNMENT;
+    const size_t base_alloc = ROUND_TO_ALIGN(sizeof(secp256k1_scratch));
     void *alloc = checked_malloc(error_callback, base_alloc + size);
     secp256k1_scratch* ret = (secp256k1_scratch *)alloc;
     if (ret != NULL) {