maint: pacify GCC 6 with -Wnull-dereference

maint: pacify GCC 6 with -Wnull-dereference

src/id.c:249:29: error: potential null pointer dereference \
[-Werror=null-dereference]
       pw_name = xstrdup (pwd->pw_name);
                          ~~~^~~~~~~~~
src/whoami.c:89:11: error: potential null pointer dereference \
[-Werror=null-dereference]
   puts (pw->pw_name);
         ~~^~~~~~~~~

* src/id.c (main): Explicitly exit with EXIT_FAILURE after an eror to
help gcc-6 to detect that the dereferenced pointer is valid.
* src/whoami.c (main): Likewise.