Panic due to nil maps (#3042)

* query/query.go: var assignment was missing map init

varValue assignments were missing map initialization for Vars
field causing panic when accessed later in fillVars().

* query/query0_test.go: added tests that found the bug

Author: srfrog

query/query.go

@@ -1495,6 +1495,7 @@ func (sg *SubGraph) populateUidValVar(doneVars map[string]varValue, sgPath []*Su
 		doneVars[sg.Params.Var] = varValue{
 			strList: sg.valueMatrix,
 			path:    sgPath,
+			Vals:    make(map[uint64]types.Val),
 		}
 	} else if len(sg.counts) > 0 {
 		// This implies it is a value variable.
@@ -1532,6 +1533,7 @@ func (sg *SubGraph) populateUidValVar(doneVars map[string]varValue, sgPath []*Su
 			doneVars[sg.Params.Var] = varValue{
 				Uids: uids,
 				path: sgPath,
+				Vals: make(map[uint64]types.Val),
 			}
 			return nil
 		}

query/query0_test.go

@@ -1728,6 +1728,37 @@ func TestCountUidToVarCombinedWithNormalVar(t *testing.T) {
 	require.JSONEq(t, `{"data": {"me":[{"score": 5}]}}`, js)
 }
 
+func TestDefaultValueVar1(t *testing.T) {
+	query := `
+	{
+		var(func: has(pred)) {
+			n as uid
+			cnt as count(_predicate_)
+		}
+
+		data(func: uid(n)) @filter(gt(val(cnt), 4)) {
+			expand(_all_)
+		}
+	}`
+	js := processQueryNoErr(t, query)
+	require.JSONEq(t, `{"data": {"data":[]}}`, js)
+}
+
+func TestDefaultValueVar2(t *testing.T) {
+	query := `
+	{
+		var(func: uid(0x1)) {
+			cnt as _predicate_
+		}
+
+		data(func: uid(0x1)) {
+			val(cnt)
+		}
+	}`
+	js := processQueryNoErr(t, query)
+	require.JSONEq(t, `{"data": {"data":[]}}`, js)
+}
+
 func TestMain(m *testing.M) {
 	populateCluster()
 	os.Exit(m.Run())