fix partial_string_tail panic (#530)

src/prolog/machine/compile.rs

@@ -348,7 +348,9 @@ fn compile_into_module(
     );
 
     match compile_into_module_impl(wam, &mut compiler, module, src, indices) {
-        Ok(()) => EvalSession::EntrySuccess,
+        Ok(()) => {
+            EvalSession::EntrySuccess
+        }
         Err(e) => {
             compiler.drop_expansions(&mut wam.code_repo);
             EvalSession::from(e)

src/prolog/machine/system_calls.rs

@@ -1154,10 +1154,19 @@ impl MachineState {
 
                 match pstr {
                     Addr::PStrLocation(h, _) => {
+                        if let HeapCellValue::PartialString(_, true) = &self.heap[h] {
                             let tail = self.heap[h + 1].as_addr(h + 1);
                             let target = self[temp_v!(2)];
 
                             self.unify(tail, target);
+                        } else {
+                            self.fail = true;
+                            return Ok(());
+                        }
+                    }
+                    Addr::EmptyList => {
+                        self.fail = true;
+                        return Ok(());
                     }
                     _ => {
                         unreachable!()