Fix the lib/libc/t_getenv:setenv_basic testcase

NetBSD checks for the `value` argument passed to setenv(3) being NULL [1] and returns -1/sets EINVAL. POSIX doesn't require this, but this requirement makes sense (setting a value to NULL doesn't make sense).

<pre>
# kyua test
t_abs:abs_basic  ->  passed  [0.006s]
t_abs:imaxabs_basic  ->  passed  [0.005s]
t_abs:labs_basic  ->  passed  [0.006s]
t_abs:llabs_basic  ->  passed  [0.006s]
t_atoi:atof_strtod  ->  passed  [0.005s]
t_atoi:atoi_strtol  ->  passed  [0.005s]
t_atoi:atol_strtol  ->  passed  [0.008s]
t_atoi:atoll_strtoll  ->  passed  [0.006s]
t_div:div_basic  ->  passed  [0.006s]
t_div:ldiv_basic  ->  passed  [0.006s]
t_div:lldiv_basic  ->  passed  [0.006s]
t_getenv:clearenv_basic  ->  passed  [0.012s]
t_getenv:getenv_basic  ->  passed  [0.006s]
t_getenv:putenv_basic  ->  passed  [0.005s]
t_getenv:setenv_basic  ->  passed  [1.477s]
t_getenv:setenv_mixed  ->  passed  [0.005s]
t_exit:exit_atexit  ->  passed  [0.006s]
t_exit:exit_basic  ->  passed  [0.006s]
t_exit:exit_status  ->  passed  [0.010s]
t_exit:exit_tmpfile  ->  passed  [0.006s]
t_posix_memalign:posix_memalign_basic  ->  passed  [0.006s]
t_random:random_same  ->  passed  [0.006s]
t_strtod:strtod_basic  ->  passed  [0.006s]
t_strtod:strtod_hex  ->  passed  [0.005s]
t_strtod:strtod_inf  ->  passed  [0.005s]
t_strtod:strtof_inf  ->  passed  [0.005s]
t_strtod:strtold_inf  ->  skipped: Requires long double support  [0.006s]
t_strtod:strtod_nan  ->  passed  [0.006s]
t_strtod:strtof_nan  ->  passed  [0.006s]
t_strtod:strtold_nan  ->  skipped: Requires long double support  [0.005s]
t_strtod:strtod_round  ->  passed  [0.006s]
t_strtod:strtod_underflow  ->  passed  [0.005s]
t_strtod:strtod_gherman_bug  ->  passed  [0.005s]
t_strtol:strtol_base  ->  passed  [0.005s]
t_strtol:strtol_case  ->  passed  [0.005s]
t_strtol:strtol_range  ->  passed  [0.005s]
t_strtol:strtol_signed  ->  passed  [0.005s]
t_system:system_basic  ->  passed  [0.016s]
t_atexit:atexit  ->  skipped: h_atexit core dumps. Isilon / WIP.  [0.022s]
t_getopt:getopt  ->  passed  [0.089s]
t_getopt:getopt_long  ->  passed  [0.091s]

41/41 passed (0 failed)
Committed action 37
</pre>

1. http://netbsd.gw.com/cgi-bin/man-cgi?setenv++NetBSD-current