make sure to unlock the screenshot's buffer on destruction

this fixes a memory leak on various devices

Bug: 8475134
Change-Id: I25924674c803a0cb22371a95c60664c36acb0340
diff --git a/libs/gui/SurfaceComposerClient.cpp b/libs/gui/SurfaceComposerClient.cpp
index ec46fce..f345df8 100644
--- a/libs/gui/SurfaceComposerClient.cpp
+++ b/libs/gui/SurfaceComposerClient.cpp
@@ -627,6 +627,10 @@
     memset(&mBuffer, 0, sizeof(mBuffer));
 }
 
+ScreenshotClient::~ScreenshotClient() {
+    ScreenshotClient::release();
+}
+
 sp<CpuConsumer> ScreenshotClient::getCpuConsumer() const {
     if (mCpuConsumer == NULL) {
         mCpuConsumer = new CpuConsumer(1);