hashtable: Fix length calculation in hexport_r

hashtable: Fix length calculation in hexport_r

The length returned by hexport_r has a few redundant characters.
This appears as NULL characters at the end so seems harmless.

Remove the surplus counts in two places

totlen += strlen(ep->key) + 2;
I'm guessing the +2 here is for = and sep char. But there is another
totlen += 2; line that does that.

size = totletn + 1;
Doesn't make sense and isn't justified with any comment.

Signed-off-by: Zubair Lutfullah Kakakhel <zubair@resin.io>