fix: Hopefully fix all tests being flaky

It seems that the varint encoding function would not work for
some integers as input. This should in theory fix this issue. Since
the SPK IDs are randomly between 0 and 2**32 - 1, it makes sense that
the tests fail only sometimes.

lib/src/omemo/sessionmanager.dart

@@ -177,15 +177,17 @@ class OmemoSessionManager {
     // Pick the correct SPK
     final device = await getDevice();
     OmemoKeyPair? spk;
-    if (kex.spkId == device.spkId) {
-      spk = device.spk;
-    } else if (kex.spkId == device.oldSpkId) {
-      spk = device.oldSpk;
-    } else {
+
+    await _lock.synchronized(() async {
+      if (kex.spkId == _device.spkId) {
+        spk = _device.spk;
+      } else if (kex.spkId == _device.oldSpkId) {
+        spk = _device.oldSpk;
+      }
+    });
+    if (spk == null) {
       throw UnknownSignedPrekeyException();
     }
-
-    assert(spk != null, 'The used SPK must be found');
     
     final kexResult = await x3dhFromInitialMessage(
       X3DHMessage(
@@ -198,7 +200,7 @@ class OmemoSessionManager {
       device.ik,
     );
     final ratchet = await OmemoDoubleRatchet.acceptNewSession(
-      spk,
+      spk!,
       OmemoPublicKey.fromBytes(kex.ik!, KeyPairType.ed25519),
       kexResult.sk,
       kexResult.ad,

lib/src/protobuf/protobuf.dart

@@ -46,21 +46,18 @@ List<int> encodeVarint(int i) {
   assert(i >= 0, "Two's complement is not implemented");
   final ret = List<int>.empty(growable: true);
 
-  var j = 0;
-  while (true) {
+  // Thanks to https://github.com/hathibelagal-dev/LEB128 for the trick with toRadixString!
+  final numSevenBlocks = (i.toRadixString(2).length / 7).ceil();
+  for (var j = 0; j < numSevenBlocks; j++) {
     // The 7 LSB of the byte we're creating
     final x = (i & (lsb7Mask << j * 7)) >> j * 7;
-    // The next bits
-    final next = i & (lsb7Mask << (j + 1) * 7);
 
-    if (next == 0) {
+    if (j == numSevenBlocks - 1) {
       // If we were to shift further, we only get zero, so we're at the end
       ret.add(x);
-      break;
     } else {
       // We still have at least one bit more to go, so set the MSB to 1
       ret.add(x + msb);
-      j++;
     }
   }