ast/parser: fix nightly fuzz case (#5330)

An unchecked ref had made it into RefHead() in one of the parsers
code paths. Now, it's rejected as it should be.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

ast/parser.go

@@ -802,6 +802,10 @@ func (p *Parser) parseHead(defaultRule bool) (*Head, bool) {
 		case Var:
 			ref = Ref{op}
 		case Ref:
+			if _, ok := y[0].Value.(Var); !ok {
+				p.illegal("rule head ref %v invalid", y)
+				return nil, false
+			}
 			ref = y
 		}
 		head = RefHead(ref)

ast/parser_test.go

@@ -2150,7 +2150,9 @@ func TestRuleRefHeads(t *testing.T) {
 		})
 	}
 
-	// TODO(sr): error cases, non-ground terms anywhere but at the end of the ref
+	assertParseErrorContains(t, "first ref head term is call", `package p
+q(0).r(0) { true }`,
+		"unexpected { token: rule head ref q(0).r invalid", opts)
 }
 
 func TestRuleElseKeyword(t *testing.T) {