refspec: initalize `refspec_item` in `valid_fetch_refspec()`

refspec: initalize `refspec_item` in `valid_fetch_refspec()`

We allocate a `struct refspec_item` on the stack without initializing
it. In particular, its `dst` and `src` members will contain some random
data from the stack. When we later call `refspec_item_clear()`, it will
call `free()` on those pointers. So if the call to `parse_refspec()` did
not assign to them, we will be freeing some random "pointers". This is
undefined behavior.

To the best of my understanding, this cannot currently be triggered by
user-provided data. And for what it's worth, the test-suite does not
trigger this with SANITIZE=address. It can be provoked by calling
`valid_fetch_refspec(":*")`.

Zero the struct, as is done in other users of `struct refspec_item` by
using the refspec_item_init() initialization function.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>