Simplify stream availability handling

Just have a simply number of bytes argument to avoid a lot of
complexity.

common/rdr/HexInStream.cxx

@@ -73,7 +73,7 @@ decodeError:
 
 
 bool HexInStream::fillBuffer(size_t maxSize, bool wait) {
-  if (!in_stream.check(2, 1, wait))
+  if (!in_stream.check(2, wait))
     return false;
 
   const U8* iptr = in_stream.getptr();