odb: shut up gcc warnings regarding uninitilized variables

The `error` variable is used as a return value in the out-section of
both `odb_read_1` and `read_prefix_1`. While the value will actually
always be initialized inside of this section, GCC fails to realize this
due to interactions with the `found` variable: if `found` is set, the
error will always be initialized. If it is not, we return early without
reaching the out-statements.

Shut up the warnings by initializing the error variable, even though it
is unnecessary.

src/odb.c

@@ -1002,7 +1002,7 @@ static int odb_read_1(git_odb_object **out, git_odb *db, const git_oid *id,
 	git_odb_object *object;
 	git_oid hashed;
 	bool found = false;
-	int error;
+	int error = 0;
 
 	if (!only_refreshed && odb_read_hardcoded(&raw, id) == 0)
 		found = true;
@@ -1099,7 +1099,7 @@ static int read_prefix_1(git_odb_object **out, git_odb *db,
 		const git_oid *key, size_t len, bool only_refreshed)
 {
 	size_t i;
-	int error;
+	int error = 0;
 	git_oid found_full_oid = {{0}};
 	git_rawobj raw = {0};
 	void *data = NULL;