PVS-Studio warning V522 for ReactOS

ir_texture *
ir_reader::read_texture(s_expression *expr)
{
  s_symbol *tag = NULL;
  ....
  } else if (MATCH(expr, other_pattern)) {
    op = ir_texture::get_opcode(tag->value());
    if (op == -1)
      return NULL;
  }
  ....
}

This suspicious code was found in ReactOS project by PVS-Studio static code analyzer.
Warning message is:
V522 Dereferencing of the null pointer 'tag' might take place. ir_reader.cpp 904

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/