fix(api): nvim_parse_cmd error message in pcall() #23297

Problem:
nvim_parse_cmd() in pcall() may show an error message (side-effect):

    :lua pcall(vim.api.nvim_parse_cmd, vim.fn.getcmdline(), {})
    E16: Invalid range

Solution:
Avoid emsg() in the nvim_parse_cmd() codepath.

- refactor(api): add error message output parameter to get_address()
- fix: null check emsg() parameter
- refactor: remove emsg_off workaround from do_incsearch_highlighting()
- refactor: remove emsg_off workaround from cmdpreview_may_show()
- refactor: remove remaining calls to emsg() from parse_cmd_address() and get_address()
- (refactor): lint set_cmd_dflall_range()
- refactor: addr_error() - move output parameter to return value

Fix #20339

TODO:

These are the functions called by `get_address()`:

```
nvim_parse_cmd() -> parse_cmdline() -> parse_cmd_address() -> get_address()
    skipwhite()
    addr_error()
    qf_get_cur_idx()
    qf_get_cur_valid_idx()
    qf_get_size()
    qf_get_valid_size()
    mark_get()
    mark_check()
    assert()
    skip_regexp()
    magic_isset()
>   do_search()
>   searchit()
    ascii_isdigit()
    getdigits()
    getdigits_int32()
    compute_buffer_local_count()
    hasFolding()
```

From these functions, I found at least two that call emsg directly:
- do_search()
  - seems to be simple to refactor
- searchit()
  - will be more challenging because it may generate multiple error messages,
    which can't be handled by the current `errormsg` out-parameter.
    For example, it makes multiple calls to `vim_regexec_multi()` in a loop that
    possibly generate error messages, and later `searchit()` itself may generate
    another one:
    - https://github.com/neovim/neovim/blob/c194acbfc479d8e5839fa629363f93f6550d035c/src/nvim/search.c#L631-L647
    - https://github.com/neovim/neovim/blob/c194acbfc479d8e5839fa629363f93f6550d035c/src/nvim/search.c#L939-L954

---------

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nvim/syntax.c b/src/nvim/syntax.c
index d237972e40..ea7f3a085f 100644
--- a/src/nvim/syntax.c
+++ b/src/nvim/syntax.c
@@ -3979,7 +3979,7 @@ static void syn_cmd_include(exarg_T *eap, int syncing)
   int sgl_id = 1;
   char *group_name_end;
   char *rest;
-  char *errormsg = NULL;
+  const char *errormsg = NULL;
   int prev_toplvl_grp;
   int prev_syn_inc_tag;
   bool source = false;