added explaination comment

Author: badasahog

src/fread.c

@@ -929,6 +929,9 @@ static void parse_double_hexadecimal(FieldParseContext *ctx)
 
   bool subnormal = false;
 
+  // Important!
+  // Keep in mind that only ch[0] is guaranteed to be mapped.
+  // Rearanging these checks will lead to segfault in rare cases.
   if (ch[0] == '0' && (ch[1] == 'x' || ch[1] == 'X') &&
       (ch[2] == '1' || (subnormal = (ch[2] == '0'))) && ch[3] == '.') {
     ch += 4;