PVS-Studio warning V600 for Firebird

class jrd_file : public pool_alloc_rpt<SCHAR, type_fil>
{
  ....
  SCHAR fil_string[1];
}

void SDW_start(....)
{
  ....
  if (dbb_file && dbb_file->fil_string &&
      expanded_name == dbb_file->fil_string)
  ....
}

This suspicious code was found in Firebird project by PVS-Studio static code analyzer.
Warning message is:
V600 Consider inspecting the condition. The 'dbb_file->fil_string' pointer is always not equal to NULL. sdw.cpp 961

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/