Fix BufferSlice self-move.

GitOrigin-RevId: 904ea326cc860dd92c49d319fc5fabde22a35223

tdutils/td/utils/buffer.h

@@ -122,6 +122,9 @@ class BufferSlice {
     debug_untrack();  // yes, debug_untrack
   }
   BufferSlice &operator=(BufferSlice &&other) {
+    if (this == &other) {
+      return *this;
+    }
     debug_untrack();
     buffer_ = std::move(other.buffer_);
     begin_ = other.begin_;

test/http.cpp

@@ -137,6 +137,8 @@ TEST(Http, reader) {
   {
     BufferSlice a("test test");
     BufferSlice b = std::move(a);
+    a = std::move(a);
+    b = std::move(b);
     a = std::move(b);
     BufferSlice c = a.from_slice(a);
     CHECK(c.size() == a.size());