make-version: use 0 as number of commits if they can't be obtained.

When running on gitlab-ci, the checkout of the project doesn't give
a complete repo. Then git describe fails at giving the number of commits.
In this case just use 0 instead of NULL.

tools/make-version.pl

@@ -188,7 +188,7 @@ sub read_repo_info {
 			if ($? == 0 && length($line) > 1) {
 				my @parts = split(/-/, $line);
 				$git_description = $line;
-				$num_commits = $parts[-2];
+				$num_commits = $parts[-2] ? $parts[-2] : "0";
 				$commit_id = $parts[-1];
 
 				if ($line =~ /v\d+\.\d+\.\d+(rc\d+)-/) {