patch 8.2.0576: some errors are not covered by tests

Problem:    Some errors are not covered by tests.
Solution:   Add a few more tests. (Dominique Pelle, closes #5920)
diff --git a/src/testdir/test_buffer.vim b/src/testdir/test_buffer.vim
index 76fb04b..de16353 100644
--- a/src/testdir/test_buffer.vim
+++ b/src/testdir/test_buffer.vim
@@ -155,4 +155,15 @@
   %bwipe!
 endfunc
 
+func Test_buffer_error()
+  new foo1
+  new foo2
+
+  call assert_fails('buffer foo', 'E93:')
+  call assert_fails('buffer bar', 'E94:')
+  call assert_fails('buffer 0', 'E939:')
+
+  %bwipe
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab