PVS-Studio warning V595 for GDCM

bool PrivateTag::ReadFromCommaSeparatedString(const char *str)
{
  unsigned int group = 0, element = 0;
  std::string owner;
  owner.resize( strlen(str) );
  if( !str || sscanf(str, "%04x,%04x,%s", &group ,
                     &element, &owner[0] ) != 3 )
  {
    gdcmDebugMacro( "Problem reading Private Tag: " << str );
    return false;
  }
  ....
}

This suspicious code was found in GDCM project by PVS-Studio static code analyzer.
Warning message is:
V595 The 'str' pointer was utilized before it was verified against nullptr. Check lines: 26, 27. gdcmprivatetag.cxx 26

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/