PVS-Studio warning V567 for Scilab

short ezxml_internal_dtd(ezxml_root_t root, char *s, size_t len)
{
  ....
  while (*(n = ++s + strspn(s, EZXML_WS)) && *n != '>') {
  ....
}

This suspicious code was found in Scilab project by PVS-Studio static code analyzer.
Warning message is:
V567 Undefined behavior. The 's' variable is modified while being used twice between sequence points. ezxml.c 385

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/