odb_pack: initialize `git_rawobj` structure

The `pack_entry_find_prefix` function receives a `git_rawobj` structure
as argument. While the function first initializes the structure to a
sensible state, Coverity is unable to correctly detect this, resulting
in a warning.

Fix this warning by initializing the object to all-zeroes before passing
it to the function.

src/odb_pack.c

@@ -428,7 +428,7 @@ static int pack_backend__read_prefix(
 			git_oid_cpy(out_oid, short_oid);
 	} else {
 		struct git_pack_entry e;
-		git_rawobj raw;
+		git_rawobj raw = {NULL};
 
 		if ((error = pack_entry_find_prefix(
 				&e, (struct pack_backend *)backend, short_oid, len)) == 0 &&