Use sizeof(*ptr) for allocating/freeing memory in forward.c

Use sizeof(*ptr) for allocating/freeing memory in forward.c

As shown in the previous commit, using sizeof(type_t) is a little
bit more error-prone when copy-pasting code, so extracting the
size information from the pointer which is being dealt with seems
like a better alternative.

(cherry picked from commit cf4003fa5801230a7e844e2abe5878520a58a748)