[arith_uint256] Avoid unnecessary this-copy using prefix operator

Author: kallewoof

src/arith_uint256.h

@@ -85,7 +85,7 @@ class base_uint
         base_uint ret;
         for (int i = 0; i < WIDTH; i++)
             ret.pn[i] = ~pn[i];
-        ret++;
+        ++ret;
         return ret;
     }