libgui: add helper to find CpuConsumer::AcquiredBuffer

Add CpuConsumer::findAcquiredBufferLocked.  Rename mBufferPointer to
mLockedBufferId.

This also fixes a nullptr-dereference when unlockBuffer is called
with a LockedBuffer whose data is nullptr.

Test: libgui_test
Change-Id: I36c665099f145731197a6ff8ab940c5d7a326df8
diff --git a/libs/gui/tests/CpuConsumer_test.cpp b/libs/gui/tests/CpuConsumer_test.cpp
index 588e541..4ef82d8 100644
--- a/libs/gui/tests/CpuConsumer_test.cpp
+++ b/libs/gui/tests/CpuConsumer_test.cpp
@@ -681,6 +681,15 @@
     }
 }
 
+TEST_P(CpuConsumerTest, FromCpuInvalid) {
+    status_t err = mCC->lockNextBuffer(nullptr);
+    ASSERT_EQ(BAD_VALUE, err) << "lockNextBuffer did not fail";
+
+    CpuConsumer::LockedBuffer b;
+    err = mCC->unlockBuffer(b);
+    ASSERT_EQ(BAD_VALUE, err) << "unlockBuffer did not fail";
+}
+
 CpuConsumerTestParams y8TestSets[] = {
     { 512,   512, 1, HAL_PIXEL_FORMAT_Y8},
     { 512,   512, 3, HAL_PIXEL_FORMAT_Y8},