Improve coverage of the test suite.

tests/test-mbitset.c

@@ -380,6 +380,8 @@ static void test_clz(void)
   }
 
   M_LET(s1, bitset_t) {
+    bitset_reset(s1);
+    assert(bitset_ctz(s1) == 0);
     bitset_set_str(s1, "[00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000]");
     assert(bitset_ctz(s1) == 116);
     bitset_set_str(s1, "[10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000]");