target/arm/kvm: Don't free migration-blocker reason on failure

The migrate_add_blocker() function documents that if it fails it will free the Error it is passed via its reasonp argument and set reasonp to NULL. That means that in kvm_arm_enable_mte() we don't need to call error_free(mte_migration_blocker) in the failure codepath. Doing so is harmless because error_free(NULL) is permitted and does nothing, but we can remove the unnecessary code. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org> Message-id: 20260223173232.453165-1-peter.maydell@linaro.org