roots: make sure we can never get a uint_max transid

If we iterate over all roots to find the max transid, but the set of
all roots is empty, we'll get a nonsense number.  Make sure that number
doesn't reach the crawling logic by killing it with an exception.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-roots.cc

@@ -229,6 +229,8 @@ BeesRoots::transid_max_nocache()
 
 	// transid must be greater than zero, or we did something very wrong
 	THROW_CHECK1(runtime_error, rv, rv > 0);
+	// transid must be less than max, or we did something very wrong
+	THROW_CHECK1(runtime_error, rv, rv < numeric_limits<uint64_t>::max());
 	return rv;
 }