Plug a mem leak in g_environ_unsetenv

Plug a mem leak in g_environ_unsetenv

And clarify the memory allocation requirement of the string arrays passed to
g_environ_{,un}setenv().

==9458== 10 bytes in 1 blocks are definitely lost in loss record 16 of 39
==9458==    at 0x402AD89: malloc (vg_replace_malloc.c:236)
==9458==    by 0x4221A1F: vasprintf (vasprintf.c:78)
==9458==    by 0x40C6065: g_vasprintf (gprintf.c:314)
==9458==    by 0x409D894: g_strdup_vprintf (gstrfuncs.c:509)
==9458==    by 0x409D8C9: g_strdup_printf (gstrfuncs.c:535)
==9458==    by 0x40672E9: g_environ_setenv (genviron.c:156)
==9458==    by 0x80490E7: test_environ_array (environment.c:78)
==9458==    by 0x40A3DB5: test_case_run (gtestutils.c:1662)
==9458==    by 0x40A40B2: g_test_run_suite_internal (gtestutils.c:1715)
==9458==    by 0x40A417C: g_test_run_suite_internal (gtestutils.c:1726)
==9458==    by 0x40A42F9: g_test_run_suite (gtestutils.c:1771)
==9458==    by 0x40A3441: g_test_run (gtestutils.c:1319)
==9458==    by 0x80493F1: main (environment.c:108)

Bug #669412.