fix(prompt): prompt_setprompt with unloaded buffer, ': with lnum 0

Problem: prompt_setprompt memory leak/other issues when fixing prompt line for
unloaded buffer, or when ': line number is zero.

Solution: don't fix prompt line for unloaded buffer. Clamp ': lnum above zero.

src/nvim/edit.c

@@ -1594,8 +1594,8 @@ static void init_prompt(int cmdchar_todo)
   int prompt_len = (int)strlen(prompt);
 
   // In case the mark is set to a nonexistent line.
-  curbuf->b_prompt_start.mark.lnum = MIN(curbuf->b_prompt_start.mark.lnum,
+  curbuf->b_prompt_start.mark.lnum = MAX(1, MIN(curbuf->b_prompt_start.mark.lnum,
-                                         curbuf->b_ml.ml_line_count);
+                                                curbuf->b_ml.ml_line_count));
 
   curwin->w_cursor.lnum = MAX(curwin->w_cursor.lnum, curbuf->b_prompt_start.mark.lnum);
   char *text = ml_get(curbuf->b_prompt_start.mark.lnum);

src/nvim/eval/buffer.c

@@ -772,9 +772,10 @@ void f_prompt_setprompt(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 
   // Update the prompt-text and prompt-marks if a plugin calls prompt_setprompt()
   // even while user is editing their input.
-  if (bt_prompt(buf)) {
+  if (bt_prompt(buf) && buf->b_ml.ml_mfp != NULL) {
     // In case the mark is set to a nonexistent line.
-    buf->b_prompt_start.mark.lnum = MIN(buf->b_prompt_start.mark.lnum, buf->b_ml.ml_line_count);
+    buf->b_prompt_start.mark.lnum = MAX(1, MIN(buf->b_prompt_start.mark.lnum,
+                                               buf->b_ml.ml_line_count));
 
     linenr_T prompt_lno = buf->b_prompt_start.mark.lnum;
     char *old_prompt = buf_prompt_text(buf);

test/functional/legacy/prompt_buffer_spec.lua

@@ -691,12 +691,19 @@ describe('prompt buffer', function()
     eq(true, api.nvim_buf_set_mark(0, ':', fn('line', '.'), 999, {}))
     eq({ 12, 6 }, api.nvim_buf_get_mark(0, ':'))
 
+    -- Clamps lnum to at least 1. Do in one event to repro the leak.
+    exec_lua(function()
+      vim.fn.setpos("':", { 0, 0, 0, 0 })
+      vim.fn.prompt_setprompt('', 'bar > ')
+    end)
+    eq({ 1, 6 }, api.nvim_buf_get_mark(0, ':'))
+
     -- No ml_get error from invalid lnum.
     command('set messagesopt+=wait:0 messagesopt-=hit-enter')
     fn('setpos', "':", { 0, 999, 7, 0 })
     eq('', api.nvim_get_vvar('errmsg'))
     command('set messagesopt&')
-    eq({ 12, 6 }, api.nvim_buf_get_mark(0, ':'))
+    eq({ 13, 6 }, api.nvim_buf_get_mark(0, ':'))
   end)
 
   describe('prompt_getinput', function()
@@ -952,5 +959,11 @@ describe('prompt buffer', function()
       {1:~                        }|*8
       {5:-- INSERT --}             |
     ]])
+
+    -- No leak if prompt_setprompt called for unloaded prompt buffer.
+    local unloaded_buf = fn('bufadd', '')
+    api.nvim_set_option_value('buftype', 'prompt', { buf = unloaded_buf })
+    fn('prompt_setprompt', unloaded_buf, 'hello unloaded! > ')
+    eq('hello unloaded! > ', fn('prompt_getprompt', unloaded_buf))
   end)
 end)