libpng: Upgrade to 1.6.26

This release contains many minor and important fixes.

Libpng 1.6.26 released on October 20, 2016

Change-Id: If647f42a1fb4d7022286010d262d289dac4030df
Signed-off-by: Alex Naidis <alex.naidis@linux.com>
diff --git a/pngwtran.c b/pngwtran.c
index 423fb2d..377b43e 100644
--- a/pngwtran.c
+++ b/pngwtran.c
@@ -1,7 +1,7 @@
 
 /* pngwtran.c - transforms the data in a row for PNG writers
  *
- * Last changed in libpng 1.6.24 [August 4, 2016]
+ * Last changed in libpng 1.6.26 [October 20, 2016]
  * Copyright (c) 1998-2002,2004,2006-2016 Glenn Randers-Pehrson
  * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
  * (Version 0.88 Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.)
@@ -177,7 +177,7 @@
    if (row_info->color_type != PNG_COLOR_TYPE_PALETTE)
    {
       int shift_start[4], shift_dec[4];
-      int channels = 0;
+      unsigned int channels = 0;
 
       if ((row_info->color_type & PNG_COLOR_MASK_COLOR) != 0)
       {