fix panic in current_prolog_flag/2 when arguments are the same variable

src/machine/system_calls.rs

@@ -9233,10 +9233,9 @@ impl Machine {
 
     #[inline(always)]
     pub(crate) fn is_sto_enabled(&mut self) {
-        self.machine_st.unify_atom(
-            self.machine_st.occurs_check.flag_value(),
-            self.machine_st.registers[1],
-        );
+        let a1 = self.deref_register(1);
+        self.machine_st
+            .unify_atom(self.machine_st.occurs_check.flag_value(), a1);
     }
 
     #[inline(always)]

tests-pl/issue2914.pl

@@ -0,0 +1,2 @@
+:- initialization(main).
+main :- ( current_prolog_flag(X, X) -> write(true) ; write(false) ).

tests/scryer/issues.rs

@@ -20,6 +20,13 @@ fn issue2588_load_html() {
     load_module_test("tests-pl/issue2588.pl", "[element(html,[],[element(head,[],[element(title,[],[[H,e,l,l,o,!]])]),element(body,[],[])])]");
 }
 
+// issue #2914
+#[test]
+#[cfg_attr(miri, ignore = "unsupported operation when isolation is enabled")]
+fn issue2914_current_prolog_flag_shared_var() {
+    load_module_test("tests-pl/issue2914.pl", "false");
+}
+
 #[test]
 #[cfg_attr(miri, ignore = "unsupported operation when isolation is enabled")]
 fn issue3256_load_xml_returns_list() {