PartitionModule: avoid nullptr crashes

Fix up iterator code so that it handles nullptr better.
This avoids part of #686.

src/modules/partition/core/PartitionCoreModule.cpp

@@ -157,9 +157,9 @@ PartitionCoreModule::doInit()
 
     // Remove the device which contains / from the list
     for ( QList< Device* >::iterator it = devices.begin(); it != devices.end(); )
-        if ( hasRootPartition( *it ) ||
+        if ( *it && ( hasRootPartition( *it ) ||
              (*it)->deviceNode().startsWith( "/dev/zram") ||
-             isIso9660( *it ) )
+             isIso9660( *it ) ) )
             it = devices.erase( it );
         else
             ++it;

src/modules/partition/core/PartitionIterator.cpp

@@ -2,6 +2,7 @@
  *
  *   Copyright 2014, Aurélien Gâteau <agateau@kde.org>
  *   Copyright 2015, Teo Mrnjavac <teo@kde.org>
+ *   Copyright 2017, Adriaan de Groot <groot@kde.org>
  *
  *   Calamares is free software: you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -83,6 +84,8 @@ PartitionIterator::operator!=( const PartitionIterator& other ) const
 PartitionIterator
 PartitionIterator::begin( Device* device )
 {
+    if ( !device )
+        return PartitionIterator( nullptr );
     Q_ASSERT(device);
     PartitionTable* table = device->partitionTable();
     if ( !table )
@@ -106,6 +109,8 @@ PartitionIterator::begin( PartitionTable* table )
 PartitionIterator
 PartitionIterator::end( Device* device )
 {
+    if ( !device )
+        return PartitionIterator( nullptr );
     PartitionTable* table = device->partitionTable();
     if ( !table )
         return PartitionIterator( nullptr );