[libpng16] Fixed cexcept.h in which GCC 5 now reports that one of the auto

variables in the Try macro needs to be volatile to prevent value
being lost over the setjmp.

pngwtran.c

diff --git a/pngwtran.c b/pngwtran.c
index 07aa97c..1a6f403 100644
--- a/pngwtran.c
+++ b/pngwtran.c
@@ -71,7 +71,8 @@
          case 2:
          {
             png_bytep sp, dp;
-            int shift, v;
+            unsigned int shift;
+            int v;
             png_uint_32 i;
             png_uint_32 row_width = row_info->width;
 
@@ -110,7 +111,8 @@
          case 4:
          {
             png_bytep sp, dp;
-            int shift, v;
+            unsigned int shift;
+            int v;
             png_uint_32 i;
             png_uint_32 row_width = row_info->width;