fix(startup): crash in read_stdin #35699

Problem:
Crash on startup in some situations due to call to `set_curbuf`
with the current value of `cur_buf`.

Solution:
Switch buffers before doing the wipeout of the stdin buffer.
Use cmdline cmds instead of raw buffer pointers to avoid lifetime issues.

test/functional/core/startup_spec.lua

@@ -536,7 +536,7 @@ describe('startup', function()
 
   it('if stdin is empty and - is last: selects buffer 1, deletes buffer 3 #35269', function()
     eq(
-      '\r\n  1 %a   "file1"                        line 0\r\n  2 #h   "file2"                        line 1',
+      '\r\n  1 %a   "file1"                        line 0\r\n  2      "file2"                        line 0',
       fn.system({
         nvim_prog,
         '-n',
@@ -554,6 +554,30 @@ describe('startup', function()
     )
   end)
 
+  it("empty stdin with terminal split doesn't crash #35681", function()
+    eq(
+      'nocrash',
+      fn.system({
+        nvim_prog,
+        '-n',
+        '-u',
+        'NONE',
+        '-i',
+        'NONE',
+        '--headless',
+        '--cmd',
+        'term',
+        '+split',
+        '+quit!',
+        '+bw!',
+        '+bw!',
+        '+echo "nocrash"',
+        "+call timer_start(1, { -> execute('qa') })", -- need to let event handling happen
+        '-',
+      }, { '' })
+    )
+  end)
+
   it('stdin with -es/-Es #7679', function()
     local input = { 'append', 'line1', 'line2', '.', '%print', '' }
     local inputstr = table.concat(input, '\n')