locales_source table grows on each request

I have both menu translations and taxonomy translations enabled. My locales_source table was getting out of control (> 500.000 rows), and I noticed it kept growing by a few hundered on every request, even if I'm just reloading the page.

I checked out the SQL queries using the Devel module, and I saw that i18nstrings_textgroup() was trying to SELECT from the 'taxonomy' textgroup, and upon not finding it, it did an INSERT into the 'default' textgroup - and so the same thing happens the next time around, won't find it with SELECT and so it will INSERT another one in the wrong place.

Attaching a small patch that fixed it for me - the table is no longer growing.