https://crbug.com/901306

commit 802d3cbf3043be5dce5317e140ccb1c17a6a2d39
Author: Thomas Bernard <miniupnp@free.fr>
Date:   Tue Jan 29 11:21:47 2019 +0100

    TIFFWriteDirectoryTagTransferfunction() : fix NULL dereferencing

    http://bugzilla.maptools.org/show_bug.cgi?id=2833

    we must check the pointer is not NULL before memcmp() the memory

diff --git a/libtiff/tif_dirwrite.c b/libtiff/tif_dirwrite.c
index c15a28dbd8fcb99b81fa5a1d44fcbcda881f42a7..ef30c869d30e210d90be16ce91f44087925fbad3 100644
--- a/libtiff/tif_dirwrite.c
+++ b/libtiff/tif_dirwrite.c
@@ -1893,12 +1893,14 @@ TIFFWriteDirectoryTagTransferfunction(TIFF* tif, uint32* ndir, TIFFDirEntry* dir
 		n=3;
 	if (n==3)
 	{
-		if (!_TIFFmemcmp(tif->tif_dir.td_transferfunction[0],tif->tif_dir.td_transferfunction[2],m*sizeof(uint16)))
+		if (tif->tif_dir.td_transferfunction[2] == NULL ||
+		    !_TIFFmemcmp(tif->tif_dir.td_transferfunction[0],tif->tif_dir.td_transferfunction[2],m*sizeof(uint16)))
 			n=2;
 	}
 	if (n==2)
 	{
-		if (!_TIFFmemcmp(tif->tif_dir.td_transferfunction[0],tif->tif_dir.td_transferfunction[1],m*sizeof(uint16)))
+		if (tif->tif_dir.td_transferfunction[1] == NULL ||
+		    !_TIFFmemcmp(tif->tif_dir.td_transferfunction[0],tif->tif_dir.td_transferfunction[1],m*sizeof(uint16)))
 			n=1;
 	}
 	if (n==0)