Don't crash milltask when no tool data file is specified

[BsAtHome]

This PR addresses a crash in milltask that happens when the ini-file lacks an entry for [EMCIO]TOOL_TABLE and a request is made to set a tool offset. The filename argument to tooldata_save() would be NULL and was naively dereferenced.

[smoe]

A bit later the filename may be assigned the value of DB_SPINDLE_SAVE. Your check may be a bit too early.

I suggest to always perform the filename[0]== 0 check (empty string test) rather than having it conditional for the !DB_ACTIVE branch and to prepend your test on NULL.

[BsAtHome]

So, you suggest:

if (db_mode == DB_ACTIVE) {
  if (!is_random_toolchanger) {return 0;}
  filename = DB_SPINDLE_SAVE; //one entry tbl (nonran only)
} else {
  if(!filename || !filename[0]) {
    UNEXPECTED_MSG;
    return -1; // Possibility one
    filename = ""; // Possibility two
  }
}

There are two possibilities to "recover". One is to return immediately with an error and two is to continue and let the fopen() fail. Having an empty filename is a distinct difference from having a NULL filename. Option two masks the actual error, filename being NULL. I don't think that is a good idea.
The second problem is that having filename NULL is actually not unexpected because it has at least one defined cause, where missing ini entry [EMCIO]TOOL_TABLE is one of them.

That would thus suggest a fix like this:

if (db_mode == DB_ACTIVE) {
  if (!is_random_toolchanger) {return 0;}
  filename = DB_SPINDLE_SAVE; //one entry tbl (nonran only)
} else {
  if(!filename) {
    fprintf(stderr, "%s: No filename. Are you missing INI-entry [EMCIO]TOOL_TABLE?",
        __PRETTY_FUNCTION__);
    return -1;
  }
  if(filename[0] == 0) {
    UNEXPECTED_MSG;
  }
}

What do you say?

[smoe]

How much do you trust DB_SPINDLE_SAVE?

include/tooldata.hh:#define DB_SPINDLE_SAVE "./db_spindle.tbl"

I would prefer to always test for the filename to be != NULL, which I think make things simpler:

if (db_mode == DB_ACTIVE) {
  if (!is_random_toolchanger) {return 0;}
  filename = DB_SPINDLE_SAVE; //one entry tbl (nonran only)
} 
if(!filename) {
  fprintf(stderr, "%s: No filename. ", __PRETTY_FUNCTION__);
  if (db_mode == DB_ACTIVE) {
     fprintf(stderr,"Internal programming error.\n");
  } else {
     fprintf(stderr,"Are you missing INI-entry [EMCIO]TOOL_TABLE?\n");
  }
  return -1;
}
if(filename[0] == 0) {
  UNEXPECTED_MSG;
}