sql: don't overwrite ctx with nil on failure

SetupLocalSyncFlow would return a nil ctx with an error in setupFlows if an
error occurred. The previous version would overwrite a ctx in the outer scope
just to return it. This commit no longer overwrites ctx and simply returns the
new ctx. On master, this nil ctx would not have any consequences although this
commit is good hygiene, but 20.2 uses this ctx in a defer, which would cause a
panic.

Release note (bug fix): Fix a nil pointer panic edge case in query setup code.

pkg/sql/distsql_running.go

@@ -224,12 +224,7 @@ func (dsp *DistSQLPlanner) setupFlows(
 	localReq := setupReq
 	localReq.Flow = *flows[thisNodeID]
 	defer physicalplan.ReleaseSetupFlowRequest(&localReq)
-	ctx, flow, err := dsp.distSQLSrv.SetupLocalSyncFlow(ctx, evalCtx.Mon, &localReq, recv, localState)
-	if err != nil {
-		return nil, nil, err
-	}
-
-	return ctx, flow, nil
+	return dsp.distSQLSrv.SetupLocalSyncFlow(ctx, evalCtx.Mon, &localReq, recv, localState)
 }
 
 // Run executes a physical plan. The plan should have been finalized using