patch 8.0.1053: setline() does not work on startup

Problem:    setline() does not work on startup. (Manuel Ortega)
Solution:   Do not check for ml_mfp to be set for the current buffer.
            (Christian Brabandt)

src/testdir/test_bufline.vim

@@ -24,3 +24,16 @@ func Test_setbufline_getbufline()
   call assert_equal([], getbufline(b, 6))
   exe "bwipe! " . b
 endfunc
+
+func Test_setline_startup()
+  let cmd = GetVimCommand('Xscript')
+  if cmd == ''
+    return
+  endif
+  call writefile(['call setline(1, "Hello")', 'w Xtest', 'q!'], 'Xscript')
+  call system(cmd)
+  call assert_equal(['Hello'], readfile('Xtest'))
+
+  call delete('Xscript')
+  call delete('Xtest')
+endfunc