td_var: avoid arithmetic on a pointer to void cast (#1096)

td_var: avoid arithmetic on a pointer to void cast (#1096)

Issue 1096 shows that a compiler complains an `arithmetic on a pointer to
void` error when `-Wpointer-arith` is turned on in Makefile. Although
there are many violating instances, we only fix the one in `parse.h`
because it is shared across many core source code files. This patch fixes
the `arithmetic on a pointer to void` issue by casting it to a uintptr_t
type first and perform manipulation, and then cast it back to void* type.

Signed-off-by: Cheng Li <chenglii@cs.rutgers.edu>