fixed an encoder bug

the bug caused the encoder to produce invalid bitstream when configured
with enable_extend_qrange.

Change-Id: I1e81c48b13359d0043cbbd480e679380a2da117c

vp8/common/entropy.c

@@ -128,7 +128,7 @@ static const Prob Pcat6[] =
 
 #endif
 
-static vp8_tree_index cat1[2], cat2[4], cat3[6], cat4[8], cat5[10], cat6[22];
+static vp8_tree_index cat1[2], cat2[4], cat3[6], cat4[8], cat5[10], cat6[26];
 
 void vp8_init_scan_order_mask()
 {