fix sql instrument with DBQueryContext (#168)

CHANGES.md

@@ -25,6 +25,7 @@ Release Notes.
 * Fix wrong tracing context when trace have been sampled.
 * Fix enhance param error when there are multiple params.
 * Fix lost trace when multi middleware `handlerFunc` in `gin` plugin.
+* Fix DBQueryContext execute error in `sql` plugin.
 
 #### Issues and PR
 - All issues are [here](https://github.com/apache/skywalking/milestone/197?closed=1)

plugins/sql/entry/span.go

@@ -54,6 +54,13 @@ func getInstanceInfo(caller interface{}) InstanceInfo {
 	if !ok || instance == nil {
 		return nil
 	}
-	info := instance.GetSkyWalkingDynamicField().(InstanceInfo)
+	df := instance.GetSkyWalkingDynamicField()
+	if df == nil {
+		return nil
+	}
+	info, ok := df.(InstanceInfo)
+	if !ok {
+		return nil
+	}
 	return info
 }