Amiga: tighten two minor issues from review

- amii_set_text_font called CloseLibrary(DiskfontBase) outside the
  OpenLibrary guard; on Kickstart V36+ that is a no-op for a NULL
  handle, but on V33/V34 it is undefined.  Move the close inside
  the if-block where DiskfontBase is known non-NULL.
- amii_get_ext_cmd's bounds check used BUFSZ for an obufp[100]
  buffer; the tighter COLNO check actually bounded it but the
  expression was misleading.  Use sizeof obufp.

sys/amiga/winami.c

@@ -499,7 +499,8 @@ amii_get_ext_cmd(void)
                 sel = com_index;
             } else {
                 colx = put_ext_cmd(obufp, colx, cw, bottom);
-                if (bufp - obufp < BUFSZ - 1 && bufp - obufp < COLNO)
+                if (bufp - obufp < (int) sizeof obufp - 1
+                    && bufp - obufp < COLNO)
                     bufp++;
             }
         } else if (c == ('X' - 64) || c == '\177') {