Remove not needed check in ibpi2str function

[ktanska]

This check in ibpi2str_table() function is probably not needed:

if (!ret) { snprintf(buf, buf_size, "(unknown: %u)", ibpi); ret = buf; }

It is only checked to put "unknown" word, but now it may be dead and the "UNKNOWN" state will be returned. To be safe we can replace this code with assertion.