cert_create: fix memory leak bug caused by key container overwrite

cert_create: fix memory leak bug caused by key container overwrite

In the current code, both key_load() and key_create() call key_new()
to allocate a key container (and they do not free it even if they
fail).  If a specific key is not given by the command option,
key_load() fails, then key_create() is called.  At this point, the
key container that has been allocated in key_load() is still alive,
and it is overwritten by a new key container created by key_create().

Move the key_new() call to the main() function to make sure it is
called just once for each descriptor.

While we are here, let's fix one more bug; the error handling code
  ERROR("Malloc error while loading '%s'\n", keys[i].fn);
is wrong because keys[i].fn is NULL pointer unless a specific key is
given by the command option.  This code could be run in either case.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>