PVS-Studio warning V557 for TortoiseGIT

static int
datasource_to_index(svn_diff_datasource_e datasource)
{
  switch (datasource)
  {
    ....
  }
  return -1;
}

static svn_error_t *
token_compare(....)
{
  ....
  int idx = datasource_to_index(file_token[i]->datasource);
  file[i] = &file_baton->files[idx];
  ....
}

This suspicious code was found in TortoiseGIT project by PVS-Studio static code analyzer.
Warning message is:
V557 Array underrun is possible. The value of 'idx' index could reach -1. diff_file.c 1052

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/