[cppcheck-fix][uninitialized-var]

This commit fixes followings cppcheck-1.59 errors:
[src/BOPAlgo/BOPAlgo_BOP.cxx:568]: (error) Uninitialized variable: iX
[src/BOPAlgo/BOPAlgo_BOP.cxx:582]: (error) Uninitialized variable: iY
[src/ViewerTest/ViewerTest.cxx:681]: (error) Uninitialized variable: aNbUIsos
[src/ViewerTest/ViewerTest.cxx:682]: (error) Uninitialized variable: aNbVIsos