com_stats(): Avoid division by zero.

If no epigrams are defined yet, the arguments to printf() contain
a division by zero which results in output like:

	average number of epigrams per file     -nan
	average number of tags per epigram.     -nan
	average number of tag recurrence...     -nan

This is not a wrong per se, but it seems to be safer to special case
this and print zero.