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, 3 insertions, 1 deletions

diff --git a/src/nvim/digraph.c b/src/nvim/digraph.c
index 243468b680..cb6bfc9cc9 100644
--- a/src/nvim/digraph.c
+++ b/src/nvim/digraph.c
@@ -1532,7 +1532,9 @@ static int getexactdigraph(int char1, int char2, int meta_char)
 
     if (convert_setup(&vc, (char_u *)"utf-8", p_enc) == OK) {
       vc.vc_fail = true;
-      to = string_convert(&vc, buf, &i);
+      assert(i >= 0);
+      size_t len = (size_t)i;
+      to = string_convert(&vc, buf, &len);
 
       if (to != NULL) {
         retval = (*mb_ptr2char)(to);