Fix null pointer issue found by fuzz test

qcode/lex.go

@@ -1,6 +1,7 @@
 package qcode
 
 import (
+	"errors"
 	"fmt"
 	"strings"
 	"unicode"
@@ -207,6 +208,9 @@ func (l *lexer) errorf(format string, args ...interface{}) stateFn {
 
 // lex creates a new scanner for the input string.
 func lex(input string) (*lexer, error) {
+	if len(input) == 0 {
+		return nil, errors.New("empty query")
+	}
 	l := &lexer{
 		input: input,
 		items: make([]item, 0, 100),

qcode/parse_test.go

@@ -57,3 +57,11 @@ func TestCompile(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestEmptyCompile(t *testing.T) {
+	qcompile, _ := NewCompiler(Config{})
+	_, err := qcompile.CompileQuery(``)
+	if err == nil {
+		t.Fatal(errors.New("expecting an error"))
+	}
+}

qcode/qcode.go

@@ -336,7 +336,13 @@ func (com *Compiler) compileQuery(op *Operation) (*Query, error) {
 		}
 	}
 
-	fil, ok := com.fm[selRoot.Table]
+	var ok bool
+	var fil *Exp
+
+	if selRoot != nil {
+		fil, ok = com.fm[selRoot.Table]
+	}
+
 	if !ok || fil == nil {
 		fil = com.fl
 	}