Fix NullPointerException in Rendering task

[sonique6784]

Hi,
here is a patch to prevent the library to crash when renderingTask is null.
I also replace "isEmpty()" and "get(0)", by an iterator.
I added some test != null to prevent any crash.

Here is the Exception: java.lang.NullPointerException: Attempt to read from field 'int com.joanzapata.pdfview.e$a.d' on a null object reference

Thank you
Cedric

PS: can you please keep up to date the maven repo so we can get the latest version through gradle ? thanks a lot

full stack below :

java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:304)
at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
at java.util.concurrent.FutureTask.run(FutureTask.java:242)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)
Caused by: java.lang.NullPointerException: Attempt to read from field 'int com.joanzapata.pdfview.e$a.d' on a null object reference
at com.joanzapata.pdfview.RenderingAsyncTask.proceed(RenderingAsyncTask.java:100)
at com.joanzapata.pdfview.RenderingAsyncTask.doInBackground(RenderingAsyncTask.java:33)
at android.os.AsyncTask$2.call(AsyncTask.java:292)
at java.util.concurrent.FutureTask.run(FutureTask.java:237)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)