libform lacks a bounds check

[john heasley]

libform of ncurses 5.5 has a constraint on the number of fields in a form
at 32767 or that of signed short.  But, Connect_Fields() does not check
it's field count (field_cnt) before sorting and such near the end of the
function.

I changed it like so, on line 185:

    182         page_nr++;
    183       fields[field_cnt]->form = form;
    184     }
    185   if (field_cnt == 0 || (short) field_cnt < -1)
    186     RETURN(E_BAD_ARGUMENT);
    187
    188   /* allocate page structures */