SPARK-2486: Utils.getCallSite is now resilient to bogus frames

When running Spark under certain instrumenting profilers,
Utils.getCallSite could crash with an NPE.  This commit
makes it more resilient to failures occurring while inspecting
stack frames.
So I was thinking that either `ste` or the result of `ste.getMethodName` could be `null` when running under instrumentation.  I'll rewrite to use checks instead of `Try`, though.