[Patch, fortran] Fix PR19101 missing & in character continuation not caught

[Paul Thomas]

Steve,

What about something like:

>
>void
>gfc_gobble_whitespace (void)
>{
>  static bool seen_tab;
>  locus old_loc;
>  int c;
>  do
>    {
>      old_loc = gfc_current_locus;
>      c = gfc_next_char_literal (0);
>      if (!gfc_option.flag_tabs && c == '\t'
>          && !seen_tab)
>        {
>           gfc_warning_now ("Nonconforming tab character at %C; "
>
                              "repetitions will be ignored.");

>           seen_tab = true;
>        }
>    }
>  while (gfc_is_whitespace (c));
>
>  gfc_current_locus = old_loc;
>}
>
>But, this can't deal with the fixed-form, column 1, issue.
>
>
It seems to me that a one-off warning to go check the source would be 
the way to do it.

Paul