handle corrupt previous answers in decodeAnswers

[bldewolf]

We got a report that a user was unable to open a problem they had previously
attempted and they were receiving this error instead:

Can't use an undefined value as an ARRAY reference at
/opt/webwork/webwork2/lib/WeBWorK/Utils.pm line 871.

The error refers to decodeAnswers, which is trying to use the result of a
Storable::thaw call without verifying that it succeeded (it returns undef on
some errors).

This change makes decodeAnswers catch those errors.

[goehle]

Thanks for submitting this fix to WeBWorK. I tested this by forcing $array_ref to be undefined. Without the patch there is a fatal error. With the patch the problem loads (without the previous answers.)

As a side note, we generally only accept pull requests into the develop branch. This change is small and so I will merge it directly into master. In the future if you could make your changes on a branch of develop and submit the pull request back into develop it would fit our workflow better.

Thanks again.