frame-system: Don't underflow the sufficients (#8062)

Closes: #8044

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: bkchr

prdoc/pr_8062.prdoc

@@ -0,0 +1,8 @@
+title: 'frame-system: Don''t underflow the sufficients'
+doc:
+- audience: Node Dev
+  description: |
+    Fixes a potential underflow for `dec_sufficients`.
+crates:
+- name: frame-system
+  bump: patch

substrate/frame/system/src/lib.rs

@@ -1693,7 +1693,7 @@ impl<T: Config> Pallet<T> {
 						DecRefStatus::Reaped
 					},
 					(x, _) => {
-						account.sufficients = x - 1;
+						account.sufficients = x.saturating_sub(1);
 						*maybe_account = Some(account);
 						DecRefStatus::Exists
 					},

substrate/frame/system/src/tests.rs

@@ -155,6 +155,14 @@ fn provider_ref_handover_to_self_sufficient_ref_works() {
 	});
 }
 
+#[test]
+fn dec_sufficients_does_not_undeflow() {
+	new_test_ext().execute_with(|| {
+		assert_eq!(System::inc_providers(&0), IncRefStatus::Created);
+		assert_eq!(System::dec_sufficients(&0), DecRefStatus::Exists);
+	});
+}
+
 #[test]
 fn self_sufficient_ref_handover_to_provider_ref_works() {
 	new_test_ext().execute_with(|| {