Enable -Wconversion: ui.c.

Refactoring summary:
- ui_write(): len: int --> size_t
    * parse_abstract_ui_codes(): len: int --> size_t
    * string_convert(): lenp: int * --> size_t *
        - string_convert_ext(): lenp      : int * --> size_t *
                                unconvlenp: int * --> size_t *
            * utf8len_tab_zero: char[] --> uint8_t[]
            * iconv_string(): slen      : int --> size_t
                              unconvlenp: int * --> size_t *
                              resultlenp: int * --> size_t *
        - mch_print_text_out(): len: int --> size_t
    * out_pos: int --> size_t

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index e3f426719e..33020c5b8c 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -578,7 +578,7 @@ Array vim_get_api_info(uint64_t channel_id)
 ///        `emsg` instead of `msg` to print each line)
 static void write_msg(String message, bool to_err)
 {
-  static int out_pos = 0, err_pos = 0;
+  static size_t out_pos = 0, err_pos = 0;
   static char out_line_buf[LINE_BUFFER_SIZE], err_line_buf[LINE_BUFFER_SIZE];
 
 #define PUSH_CHAR(i, pos, line_buf, msg)                                      \