Refs #10710 Fix uninitialized scalar variable

It's an uninitialized scalar variable in code I wrote. It's an easy fix, and Coverity's flagged its impact as high, so it may as well be taken care of. 

http://trac.mantidproject.org/mantid/ticket/10710

**Testing**
This is a trivial change. Code inspection + verifying build has not been broken ought to be sufficient.