gralloc4-vts: don't wait on invalid sync fence

Sync fences can be invalid if the buffer is already ready. If the
fence is invalid, we do not need to wait.

Test: VtsHalGraphicsMapperV4_0
Bug: 150213134

Change-Id: I8326d5f8b358c466ee9a686fd299daed1d4b4aa8
diff --git a/graphics/mapper/4.0/vts/functional/VtsHalGraphicsMapperV4_0TargetTest.cpp b/graphics/mapper/4.0/vts/functional/VtsHalGraphicsMapperV4_0TargetTest.cpp
index 1416fcc..5912c3e 100644
--- a/graphics/mapper/4.0/vts/functional/VtsHalGraphicsMapperV4_0TargetTest.cpp
+++ b/graphics/mapper/4.0/vts/functional/VtsHalGraphicsMapperV4_0TargetTest.cpp
@@ -725,8 +725,10 @@
 
     int fence;
     ASSERT_NO_FATAL_FAILURE(fence = mGralloc->flushLockedBuffer(writeBufferHandle));
-    ASSERT_EQ(0, sync_wait(fence, 3500));
-    close(fence);
+    if (fence >= 0) {
+        ASSERT_EQ(0, sync_wait(fence, 3500));
+        close(fence);
+    }
 
     ASSERT_NO_FATAL_FAILURE(mGralloc->rereadLockedBuffer(readBufferHandle));