[assert] fix set-but-unused warnings (#10823)

src/core/thread/mlr_manager.cpp

@@ -719,6 +719,8 @@ void MlrManager::CheckInvariants(void) const
 #if OPENTHREAD_EXAMPLES_SIMULATION && OPENTHREAD_CONFIG_ASSERT_ENABLE
     uint16_t registeringNum = 0;
 
+    OT_UNUSED_VARIABLE(registeringNum);
+
     OT_ASSERT(!mMlrPending || mSendDelay == 0);
 
 #if OPENTHREAD_CONFIG_MLR_ENABLE

src/lib/spinel/spinel_buffer.cpp

@@ -784,6 +784,8 @@ otError Buffer::OutFrameRemove(void)
     uint8_t  numSegments;
     FrameTag tag;
 
+    OT_UNUSED_VARIABLE(numSegments);
+
     EXPECT(!IsEmpty(), error = OT_ERROR_NOT_FOUND);
 
     OutFrameSelectReadDirection();
@@ -884,6 +886,8 @@ uint16_t Buffer::OutFrameGetLength(void)
     otMessage *message = nullptr;
 #endif
 
+    OT_UNUSED_VARIABLE(numSegments);
+
     // If the frame length was calculated before, return the previously calculated length.
     EXPECT(mReadFrameLength == kUnknownFrameLength, frameLength = mReadFrameLength);