checkout: set ignorecase=0 when config lookup fails

When `git_repository__cvar` fails we may end up with a
`ignorecase` value of `-1`. As we subsequently check if
`ignorecase` is non-zero, we may end up reporting that data
should be removed when in fact it should not.

Err on the safer side and set `ignorecase = 0` when
`git_repository__cvar` fails.

src/checkout.c

@@ -1342,9 +1342,11 @@ fail:
 
 static bool should_remove_existing(checkout_data *data)
 {
-	int ignorecase = 0;
+	int ignorecase;
 
-	git_repository__cvar(&ignorecase, data->repo, GIT_CVAR_IGNORECASE);
+	if (git_repository__cvar(&ignorecase, data->repo, GIT_CVAR_IGNORECASE) < 0) {
+		ignorecase = 0;
+	}
 
 	return (ignorecase &&
 		(data->strategy & GIT_CHECKOUT_DONT_REMOVE_EXISTING) == 0);