cmdgen: fix double-free on exit.

Freeing ssh1key->comment before calling freersakey() on the whole of
ssh1key is redundant, and worse, because we also didn't null out the
freed pointer, causes a double-free.

cmdgen.c

@@ -1064,7 +1064,6 @@ int main(int argc, char **argv)
     }
 
     if (ssh1key) {
-	sfree(ssh1key->comment);
 	freersakey(ssh1key);
     }
     if (ssh2key) {