Fix an integer wrap around bug and add a couple more tests

src/megolm.c

@@ -116,8 +116,12 @@ void megolm_advance_to(Megolm *megolm, uint32_t advance_to) {
             ((advance_to >> shift) - (megolm->counter >> shift)) & 0xff;
 
         if (steps == 0) {
+            if (advance_to < megolm->counter) {
+                steps = 0x100;
+            } else {
                 continue;
             }
+        }
 
         /* for all but the last step, we can just bump R(j) without regard
          * to R(j+1)...R(3).

tests/test_megolm.cpp

@@ -98,4 +98,37 @@ std::uint8_t random_bytes[] =
     assert_equals(megolm_get_data(&mr2), megolm_get_data(&mr1), MEGOLM_RATCHET_LENGTH);
 }
 
+{
+    TestCase test_case("Megolm::advance overflow by one");
+
+    Megolm mr1, mr2;
+
+    megolm_init(&mr1, random_bytes, 0xffffffffUL);
+    megolm_advance_to(&mr1, 0x0);
+    assert_equals(0x0U, mr1.counter);
+
+    megolm_init(&mr2, random_bytes, 0xffffffffUL);
+    megolm_advance(&mr2);
+    assert_equals(0x0U, mr2.counter);
+
+    assert_equals(megolm_get_data(&mr2), megolm_get_data(&mr1), MEGOLM_RATCHET_LENGTH);
+}
+
+{
+    TestCase test_case("Megolm::advance overflow");
+
+    Megolm mr1, mr2;
+
+    megolm_init(&mr1, random_bytes, 0x1UL);
+    megolm_advance_to(&mr1, 0x80000000UL);
+    megolm_advance_to(&mr1, 0x0);
+    assert_equals(0x0U, mr1.counter);
+
+    megolm_init(&mr2, random_bytes, 0x1UL);
+    megolm_advance_to(&mr2, 0x0UL);
+    assert_equals(0x0U, mr2.counter);
+
+    assert_equals(megolm_get_data(&mr2), megolm_get_data(&mr1), MEGOLM_RATCHET_LENGTH);
+}
+
 }