libgui: Clear frame number while freeing slot

Clears the frame number of a slot when it is freed, since it is used
to determine if a released buffer is stale.

Bug: 20445852
Change-Id: I02415e7b25a1eafe7414d6eb1cedf62ac5543cd9
diff --git a/libs/gui/BufferQueueConsumer.cpp b/libs/gui/BufferQueueConsumer.cpp
index c7d5e00..c3e1019 100644
--- a/libs/gui/BufferQueueConsumer.cpp
+++ b/libs/gui/BufferQueueConsumer.cpp
@@ -292,6 +292,8 @@
 
     if (slot < 0 || slot >= BufferQueueDefs::NUM_BUFFER_SLOTS ||
             releaseFence == NULL) {
+        BQ_LOGE("releaseBuffer: slot %d out of range or fence %p NULL", slot,
+                releaseFence.get());
         return BAD_VALUE;
     }
 
@@ -330,7 +332,7 @@
             mSlots[slot].mNeedsCleanupOnRelease = false;
             return STALE_BUFFER_SLOT;
         } else {
-            BQ_LOGV("releaseBuffer: attempted to release buffer slot %d "
+            BQ_LOGE("releaseBuffer: attempted to release buffer slot %d "
                     "but its state was %d", slot, mSlots[slot].mBufferState);
             return BAD_VALUE;
         }