Lock when reading a ConflatedChannel's value. (#3248)

Reads in `isBufferEmpty()` were detected as a race by internal tests.

The relevant stack snippets are:

```
  Read of size 4 at 0x0000cf809644 by thread T10 (mutexes: write M0, write M1):
    #0 kotlinx.coroutines.channels.ConflatedChannel.isBufferEmpty()Z ConflatedChannel.kt:22
```

and:

```
  Previous write of size 4 at 0x0000cf809644 by thread T32 (mutexes: write M2, write M3, write M4, write M5, write M6, write M7):
    #0 kotlinx.coroutines.channels.ConflatedChannel.updateValueLocked(Ljava/lang/Object;)Lkotlinx/coroutines/internal/UndeliveredElementException; ConflatedChannel.kt:131
```

Author: mpetrov

kotlinx-coroutines-core/common/src/channels/ConflatedChannel.kt

@@ -19,7 +19,7 @@ import kotlinx.coroutines.selects.*
  */
 internal open class ConflatedChannel<E>(onUndeliveredElement: OnUndeliveredElement<E>?) : AbstractChannel<E>(onUndeliveredElement) {
     protected final override val isBufferAlwaysEmpty: Boolean get() = false
-    protected final override val isBufferEmpty: Boolean get() = value === EMPTY
+    protected final override val isBufferEmpty: Boolean get() = lock.withLock { value === EMPTY }
     protected final override val isBufferAlwaysFull: Boolean get() = false
     protected final override val isBufferFull: Boolean get() = false
 
@@ -139,5 +139,5 @@ internal open class ConflatedChannel<E>(onUndeliveredElement: OnUndeliveredEleme
     // ------ debug ------
 
     override val bufferDebugString: String
-        get() = "(value=$value)"
+        get() = lock.withLock { "(value=$value)" }
 }