vim-patch:8.2.4901: NULL pointer access when using invalid pattern

Problem:    NULL pointer access when using invalid pattern.
Solution:   Check for failed regexp program.
8e4b76da1d

src/nvim/testdir/test_buffer.vim

@@ -61,4 +61,11 @@ func Test_buffer_scheme()
   set shellslash&
 endfunc
 
+" this was using a NULL pointer after failing to use the pattern
+func Test_buf_pattern_invalid()
+  vsplit 0000000
+  silent! buf [0--]\&\zs*\zs*e
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab