refactor(tui): check for out of bound access after snprintf (#24751)

Counterintuitively, snprintf returns the number of characters it _should
have written_ if it had not encoutered the length bound, thus leading to
a potential buffer overflow.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/nvim/tui/input.c

@@ -297,10 +297,10 @@ static void forward_simple_utf8(TermInput *input, TermKeyKey *key)
     } else {
       buf[len++] = *ptr;
     }
+    assert(len < sizeof(buf));
     ptr++;
   }
 
-  assert(len < sizeof(buf));
   tinput_enqueue(input, buf, len);
 }