vim-patch:8.2.0426: some errors were not tested for

Problem:    Some errors were not tested for.
Solution:   Add tests. (Dominique Pelle, closes vim/vim#5824)
9b9be007e7

Cherry-pick get_highest_fnum() from patch 8.1.1908 to make tests pass.

src/nvim/ex_docmd.c

@@ -4616,8 +4616,9 @@ char *invalid_range(exarg_T *eap)
       }
       break;
     case ADDR_BUFFERS:
-      if (eap->line1 < firstbuf->b_fnum
-          || eap->line2 > lastbuf->b_fnum) {
+      // Only a boundary check, not whether the buffers actually
+      // exist.
+      if (eap->line1 < 1 || eap->line2 > get_highest_fnum()) {
         return _(e_invrange);
       }
       break;