check: enforce correct alignment during SNMP tests

check: enforce correct alignment during SNMP tests

This should fix the following clang error, despite the fact that we know
that the alignment was correct:

```
check_snmp.c:900:9: fatal error: cast from 'u_char *' (aka 'unsigned char *') to
      'unsigned long *' increases required alignment from 1 to 8 [-Wcast-align]
                            *(unsigned long int *)result,
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/check.h:220:22: note: expanded from macro 'fail_unless'
        _fail_unless(expr, __FILE__, __LINE__,\
                     ^
1 error generated.
```