Fix NPE in PAGImageView.flush() by moving bitmapCache.put() inside the bitmapLock synchronized block. (#3429)

Author: leiyue123

android/libpag/src/main/java/org/libpag/PAGImageView.java

@@ -679,9 +679,9 @@ private boolean handleFrame(final int frame) {
                 flushBitmap.prepareToDraw();
             }
             renderBitmap = flushBitmap;
-        }
-        if (_cacheAllFramesInMemory && renderBitmap != null) {
-            bitmapCache.put(frame, renderBitmap);
+            if (_cacheAllFramesInMemory && flushBitmap != null) {
+                bitmapCache.put(frame, flushBitmap);
+            }
         }
         return true;
     }