BUG: Fix uninitialized m_Spacing in SignedMaurerDistance

If PrintSelf was called before execution not value had been assigned
to the m_Spacing variable. This defect was detected by Valgrind on the
SimpleITK dashboard.

Change-Id: I206f9c6fb4fb4d524922e3fcf973051f3f165769
Merge topic 'FixUninitializedMaurerDistanceIVar'

53bcb4c BUG: Fix uninitialized m_Spacing in SignedMaurerDistance