parser: Fix missing early return on index error

Add missing early return on index expression parse error. This resulted
in a "dereferenced nil pointer" panic at run time.

Author: juliaogris

pkg/parser/expression.go

@@ -144,6 +144,9 @@ func (p *Parser) parserIndexExpr(scope *scope, left Node) Node {
 		return nil
 	}
 	index := p.parseTopLevelExpr(scope)
+	if index == nil {
+		return nil
+	}
 	if !p.assertToken(lexer.RBRACKET) {
 		return nil
 	}

pkg/parser/expression_test.go

@@ -67,6 +67,7 @@ func TestParseTopLevelExpression(t *testing.T) {
 		`print (len "abc") 2`:         "print(len('abc'), 2)",
 		`print (len "abc") (len "x")`: "print(len('abc'), len('x'))",
 		`print s[1]`:                  "print((s[1]))",
+		"print map2[s]":               "print((map2[s]))",
 
 		// Index expression
 		"arr[1]":        "(arr[1])",
@@ -88,6 +89,7 @@ func TestParseTopLevelExpression(t *testing.T) {
 		"map3.ok[n1]":      "((map3.ok)[n1])",
 		"list[1].x":        "((list[1]).x)",
 		"list[n1][s]":      "((list[n1])[s])",
+		"map2[s]":          "(map2[s])",
 
 		// Array literals
 		"[]":          "[]",

pkg/parser/parser_test.go

@@ -79,6 +79,10 @@ func TestParseDeclarationError(t *testing.T) {
 		"a := {}[":      "line 1 column 9: unexpected end of input",
 		"a :num num":    "line 1 column 8: expected end of line, found 'num'",
 		"a :num{}num":   "line 1 column 7: expected end of line, found '{'",
+		`
+m := {name: "Greta"}
+s := name
+print m[s]`: "line 3 column 6: unknown variable name 'name'",
 	}
 	for input, err1 := range tests {
 		parser := New(input, testBuiltins())