repart: mark an assert()-only variable as unused

repart: mark an assert()-only variable as unused

otherwise compilation with -Db_ndebug=true complains about a
set-but-not-used variable:

```
 ../src/partition/repart.c:907:33: error: variable 'left' set but not used [-Werror,-Wunused-but-set-variable]
                 uint64_t start, left;
                                                 ^
                                                 1 error generated.
```