Fix a warning about undefined behavior.

I get an error of the form
  /u/bangerth/p/deal.II/1/dealii/include/deal.II/meshworker/dof_info.h:69:9: runtime error: load of value 216, which is not a valid value for type 'bool'
when running the testsuite with the ASAN framework. This is caused by the DoFInfo
class having a member variable that is left uninitialized in the constructor.
Fix this.
Merge pull request #101 from bangerth/fix-uninitialized-variable

Fix a warning about undefined behavior.