Fix hex_dump..was printing negative values

src/debug.c

@@ -84,7 +84,7 @@ void dump_hex(const unsigned char ptr[], size_t size)
 
     for (i = 0; i < size; ++i)
     {
-        char c;
+        unsigned char c;
         if (i % DUMP_HEX_WIDTH == 0)
         {
             /* new line */