fix(coverity): INTEGER_OVERFLOW #31657

CID 516419:  Integer handling issues  (INTEGER_OVERFLOW)
    /src/nvim/message.c: 2242 in msg_puts_display()
    2236         }
    2237         // Concat pieces with the same highlight
    2238         size_t len = maxlen < 0 ? strlen(str) : strnlen(str, (size_t)maxlen);
    2239         ga_concat_len(&msg_ext_last_chunk, str, len);
    2240         msg_ext_cur_len += len;
    2241         // When message ends in newline, reset variables used to format message: msg_advance().
    >>>     CID 516419:  Integer handling issues  (INTEGER_OVERFLOW)
    >>>     Expression "len - 1UL", which is equal to 18446744073709551615, where "len" is known to be equal to 0, underflows the type that receives it, an unsigned integer 64 bits wide.
    2242         if (str[len - 1] == '\n') {
    2243           msg_ext_cur_len = 0;
    2244           msg_col = 0;
    2245         }
    2246         return;
    2247       }

src/nvim/message.c

@@ -2239,6 +2239,7 @@ static void msg_puts_display(const char *str, int maxlen, int hl_id, int recurse
     ga_concat_len(&msg_ext_last_chunk, str, len);
     msg_ext_cur_len += len;
     // When message ends in newline, reset variables used to format message: msg_advance().
+    assert(len > 0);
     if (str[len - 1] == '\n') {
       msg_ext_cur_len = 0;
       msg_col = 0;