Update bn_mp_init_copy.c

In mp_init_copy(), mp_init() is used to assign the memory with default block size, mp_copy() is used to grow the target mp_int size if it is required and copy the content.

My suggestion is to use mp_init_size() instead of mp_init() inside the mp_init_copy to assign required memory depending on the source mp_int size and then use mp_copy to copy the content. This will avoid the subsequent mp_grow() inside the mp_copy(). 

The associated issue number is 11 in GitHub.

bn_mp_init_copy.c

@@ -20,7 +20,7 @@ int mp_init_copy (mp_int * a, mp_int * b)
 {
   int     res;
 
-  if ((res = mp_init (a)) != MP_OKAY) {
+  if ((res = mp_init_size (a, b->used)) != MP_OKAY) {
     return res;
   }
   return mp_copy (b, a);