Fix a crash when opening an empty file.

[ruevs]

Fixes: #918

The problem was that here:

solvespace/src/file.cpp

Line 480 in 11a8a0a

ClearExisting();

we cleared all groups. Then we tried to read an empty file and therefore this while exited immediately:

solvespace/src/file.cpp

Line 487 in 11a8a0a

while(fgets(line, (int)sizeof(line), fh)) {

and we came to here:

solvespace/src/file.cpp

Line 548 in 11a8a0a

if(fileLoadError) {

where there was no fileLoadError and thus we were left with no groups, and so this assert failed:

solvespace/src/graphicswin.cpp

Line 394 in 11a8a0a

"Group order can't be empty since we will activate the last group.");

since is is not allowed to have no groups :-)

[phkahler]

I'd prefer "The file is empty." But it's OK either way.

[ruevs]

You mean remove "it may be corrupt"?
My logic was that a normal user should normally not have a zero length slvs file, so for them " the file is empty" may be an unpleasant surprise. Not that telling them it may be corrupt will help :-)

[rpavlik]

I'm not sure when this message would actually appear - just when e.g. the user has a complete 0 byte slvs file? As such I have no opinion on the strings wording.

That said, the code itself looks reasonable.

[ruevs]

I'm not sure when this message would actually appear - just when e.g. the user has a complete 0 byte slvs file?

Yes, only on a 0 byte file.
Only then the first fgets would return 0 (a null pointer).

"If the end-of-file is encountered while attempting to read a character, the eof indicator is set (feof). If this happens before any characters could be read, the pointer returned is a null pointer (and the contents of str remain unchanged)."