Remove 'inst != NULL' from cleanup check in plugin_register

Remove 'inst != NULL' from cleanup check in plugin_register

'inst' is guarenteed to be non NULL at this point.

    358        *instp = inst;
    359
    360cleanup:

    CID 281450 (#2 of 2): Dereference before null check (REVERSE_INULL)
    check_after_deref: Null-checking inst suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
    361        if (result != ISC_R_SUCCESS && inst != NULL) {
    362                plugin_destroy((void **)&inst);
    363        }
    364
    365        return (result);

(cherry picked from commit 8c5539e905aec0e91e0bb1b390be8edb2e558c11)