[Spark SQL] SQLContext getOrCreate incorrect behaviour

[Jerry Lam]

Hi Spark developers,

I found that SQLContext.getOrCreate(sc: SparkContext) does not behave
correctly when a different spark context is provided.

```
val sc = new SparkContext
val sqlContext =SQLContext.getOrCreate(sc)
sc.stop
...

val sc2 = new SparkContext
val sqlContext2 = SQLContext.getOrCreate(sc2)
sc2.stop
```

The sqlContext2 will reference sc instead of sc2 and therefore, the program
will not work because sc has been stopped.

Best Regards,

Jerry