X.509: fix buffer overflow detection in sprint_oid()

In sprint_oid(), if the input buffer were to be more than 1 byte too
small for the first snprintf(), 'bufsize' would underflow, causing a
buffer overflow when printing the remainder of the OID.

Fortunately this cannot actually happen currently, because no users pass
in a buffer that can be too small for the first snprintf().

Regardless, fix it by checking the snprintf() return value correctly.

For consistency also tweak the second snprintf() check to look the same.

Fixes: 4f73175d ("X.509: Add utility functions to render OIDs as strings")
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>