Fixing uses of AutoPtr in MOOSE for upcoming libmesh refactoring.

This patch does not require a libmesh update to work, it only enforces
proper AutoPtr usage.

Refs #3798.
It is ambiguous to construct a std::unique_ptr with an explicit (NULL) argument.

This is apparently because there is a 'nullptr' constructor for this
class.  Luckily, using the default constructor instead works just fine.

Refs #3799.
Merge pull request #3799 from jwpeterson/fix_autoptr

Fixing uses of AutoPtr in MOOSE for upcoming libmesh refactoring.