add -p: fix memory leak

add -p: fix memory leak

asan reports that the C version of `add -p` is not freeing all the
memory it allocates. Fix this by introducing a function to clear
`struct add_p_state` and use it instead of freeing individual members.

Signed-off-by: Phillip Wood <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>