fix(api): fix crash in command preview with % #35228

Problem: parse_cmdline() sets eap->cmdlinep to address of local parameter,
causing invalid memory access when expand_filename() tries to modify it.
This leads to crashes when typing '%' in user commands with preview=true
and complete=file.

Solution: Change parse_cmdline() signature to accept char **cmdline,
allowing cmdlinep to point to caller's variable for safe reallocation.

src/nvim/ex_getln.c

@@ -2678,7 +2678,7 @@ static bool cmdpreview_may_show(CommandLineState *s)
   char *cmdline = xstrdup(ccline.cmdbuff);
   const char *errormsg = NULL;
   emsg_off++;  // Block errors when parsing the command line, and don't update v:errmsg
-  if (!parse_cmdline(cmdline, &ea, &cmdinfo, &errormsg)) {
+  if (!parse_cmdline(&cmdline, &ea, &cmdinfo, &errormsg)) {
     emsg_off--;
     goto end;
   }