Prevents panic in some strange cases. Seen in #721

crates/prolog_parser/src/parser.rs

@@ -481,6 +481,10 @@ impl<'a, R: Read> Parser<'a, R> {
             return false;
         }
 
+        if self.terms.len() < 1 + arity {
+            return false;
+        }
+
         let stack_len = self.stack.len() - 2 * arity - 1;
         let idx = self.terms.len() - arity;
 
@@ -638,6 +642,13 @@ impl<'a, R: Read> Parser<'a, R> {
             term
         };
 
+        if arity > self.terms.len() {
+            return Err(ParserError::IncompleteReduction(
+                self.lexer.line_num,
+                self.lexer.col_num
+            ))
+        }
+
         let idx = self.terms.len() - arity;
 
         let list = self.terms.drain(idx..).rev().fold(end_term, |acc, t| {