Silence CID 281450: Dereference before null check

Silence CID 281450: Dereference before null check

remove redundant 'inst != NULL' test

    162cleanup:

    CID 281450 (#1 of 1): 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.
    163        if (result != ISC_R_SUCCESS && inst != NULL) {
    164                plugin_destroy((void **)&inst);
    165        }