Fix hIST chunk position to match W3C specification

The hIST chunk was incorrectly positioned in the code. According to the
W3C specification, the hIST chunk must appear before the first IDAT
chunk and after PLTE (if present). This commit corrects the chunk
positioning to comply with the specification.

pngrutil.c

diff --git a/pngrutil.c b/pngrutil.c
index c05fa1c..39567b8 100644
--- a/pngrutil.c
+++ b/pngrutil.c
@@ -3259,7 +3259,7 @@
 #  define CDiTXt  NoCheck,    6U,      0, hIHDR,        1
       /* Allocates 'length+1'; checked in the handler */
 #  define CDbKGD       6U,    1U,  hIDAT, hIHDR,        0
-#  define CDhIST    1024U,    0U,  hPLTE, hIHDR,        0
+#  define CDhIST    1024U,    0U,  hIDAT, hPLTE,        0
 #  define CDpHYs       9U,    9U,  hIDAT, hIHDR,        0
 #  define CDsPLT  NoCheck,    3U,  hIDAT, hIHDR,        1
       /* Allocates 'length+1'; checked in the handler */