Memory leak bug fix in --key-gen mode

Memory leak bug fix in --key-gen mode

This commit fixes the following memory caught with the test suite in valgrind
mode:

HEAP SUMMARY:
    in use at exit: 285 bytes in 4 blocks
  total heap usage: 11 allocs, 7 frees, 3,179 bytes allocated

5 bytes in 1 blocks are indirectly lost in loss record 1 of 4
   at 0x4C2B6CD: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x50CB801: strdup (strdup.c:43)
   by 0x4E3A7B2: fko_set_username (fko_user.c:96)
   by 0x4E39628: fko_new (fko_funcs.c:86)
   by 0x10AB54: main (fwknop.c:83)

7 bytes in 1 blocks are indirectly lost in loss record 2 of 4
   at 0x4C2B6CD: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4E395D7: fko_new (fko_funcs.c:62)
   by 0x10AB54: main (fwknop.c:83)

17 bytes in 1 blocks are indirectly lost in loss record 3 of 4
   at 0x4C2B6CD: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4E3A06A: fko_set_rand_value (fko_rand_value.c:114)
   by 0x4E39605: fko_new (fko_funcs.c:75)
   by 0x10AB54: main (fwknop.c:83)

285 (256 direct, 29 indirect) bytes in 1 blocks are definitely lost in loss record 4 of 4
   at 0x4C29DB4: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4E395BA: fko_new (fko_funcs.c:46)
   by 0x10AB54: main (fwknop.c:83)

LEAK SUMMARY:
   definitely lost: 256 bytes in 1 blocks
   indirectly lost: 29 bytes in 3 blocks
     possibly lost: 0 bytes in 0 blocks
   still reachable: 0 bytes in 0 blocks
        suppressed: 0 bytes in 0 blocks