FPII-2034: DO NOT MERGE Update libpng to 1.6.20

DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I2fc2106965c789a54f04412e9c63a0901b0c1e71
(cherry picked from commit 0b39f202aaedd3029e4270d2af7d58ba6634f9cd)
diff --git a/tests/pngunknown-if-safe b/tests/pngunknown-if-safe
index 7c52824..31a4954 100755
--- a/tests/pngunknown-if-safe
+++ b/tests/pngunknown-if-safe
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngunknown default=if-safe "${srcdir}/pngtest.png"
+exec ./pngunknown --strict default=if-safe "${srcdir}/pngtest.png"