(imap_writeline): Fix handling of input

lines longer than 254 chars. Proposed by Daniel Kahn Gillmor.

@@ -2023,7 +2023,7 @@ imap_writeline (f_imap_t f_imap, const char *format, ...)
   va_start(ap, format);
   do
     {
-      len = vsnprintf (f_imap->buffer, f_imap->buflen - 1, format, ap);
+      len = vsnprintf (f_imap->buffer, f_imap->buflen, format, ap);
       if (len < 0 || len >= (int)f_imap->buflen
           || !memchr (f_imap->buffer, '\0', len + 1))
         {