app/vminsert: properly initialize InsertCtx

This should prevent from panic described at https://github.com/VictoriaMetrics/VictoriaMetrics/issues/339

app/vminsert/graphite/request_handler.go

@@ -32,6 +32,7 @@ func insertRows(at *auth.Token, rows []parser.Row) error {
 	ctx := netstorage.GetInsertCtx()
 	defer netstorage.PutInsertCtx(ctx)
 
+	ctx.Reset() // This line is required for initializing ctx internals.
 	atCopy := *at
 	for i := range rows {
 		r := &rows[i]

app/vminsert/influx/request_handler.go

@@ -59,6 +59,7 @@ func insertRows(at *auth.Token, db string, rows []parser.Row) error {
 	defer putPushCtx(ctx)
 
 	ic := &ctx.Common
+	ic.Reset() // This line is required for initializing ic internals.
 	rowsTotal := 0
 	for i := range rows {
 		r := &rows[i]

app/vminsert/opentsdb/request_handler.go

@@ -32,6 +32,7 @@ func insertRows(at *auth.Token, rows []parser.Row) error {
 	ctx := netstorage.GetInsertCtx()
 	defer netstorage.PutInsertCtx(ctx)
 
+	ctx.Reset() // This line is required for initializing ctx internals.
 	atCopy := *at
 	for i := range rows {
 		r := &rows[i]

app/vminsert/opentsdbhttp/request_handler.go

@@ -50,6 +50,7 @@ func insertRows(at *auth.Token, rows []parser.Row) error {
 	ctx := netstorage.GetInsertCtx()
 	defer netstorage.PutInsertCtx(ctx)
 
+	ctx.Reset() // This line is required for initializing ctx internals.
 	for i := range rows {
 		r := &rows[i]
 		ctx.Labels = ctx.Labels[:0]

app/vminsert/promremotewrite/request_handler.go

@@ -31,6 +31,7 @@ func insertRows(at *auth.Token, timeseries []prompb.TimeSeries) error {
 	ctx := netstorage.GetInsertCtx()
 	defer netstorage.PutInsertCtx(ctx)
 
+	ctx.Reset() // This line is required for initializing ctx internals.
 	rowsTotal := 0
 	for i := range timeseries {
 		ts := &timeseries[i]

app/vminsert/vmimport/request_handler.go

@@ -32,6 +32,7 @@ func insertRows(at *auth.Token, rows []parser.Row) error {
 	ctx := netstorage.GetInsertCtx()
 	defer netstorage.PutInsertCtx(ctx)
 
+	ctx.Reset() // This line is required for initializing ctx internals.
 	rowsTotal := 0
 	for i := range rows {
 		r := &rows[i]