vector_push_back  NULL ptr,   abort in vector_destroy

vector_push_back  null crash in vector_destroy

```
    vector_t *pvec_charptr = create_vector(char *);
    vector_init(pvec_charptr);
    char *p;
    char *q = NULL;
    vector_push_back(pvec_charptr, p);
    vector_push_back(pvec_charptr, q);
    vector_destroy(pvec_charptr);
```
