#250 %n expansion can lead to core dump due to NULL dereference

[nasm64developer]

Once both SF #872335 and SF #1284169 have been
fixed, expand_mmac_params() may require a

if ( !tt ) {
break;
}

check at the bottom of the for() loop which is
handling the expansion of %n (=default case),
to prevent potential tt=NULL dereferences that
cause a core dump.