utf16_to_utf8: align with libuv

- take a size parameter
- return libuv error code
- handle error in caller only (avoid redundant messages)

53995a3825
4c945f4936

src/nvim/main.c

@@ -237,7 +237,7 @@ int main(int argc, char **argv)
   char **argv = xmalloc((size_t)argc * sizeof(char *));
   for (int i = 0; i < argc; i++) {
     char *buf = NULL;
-    utf16_to_utf8(argv_w[i], &buf);
+    utf16_to_utf8(argv_w[i], -1, &buf);
     assert(buf);
     argv[i] = buf;
   }