Fix coverity issues with CompareMDWorkspaces

[sf1919]

Describe the outcome that is desired. CompareMDWorkspaces.cpp (https://github.com/mantidproject/mantid/blob/main/Framework/MDAlgorithms/src/CompareMDWorkspaces.cpp) will no longer have any Coverity issues.

Describe any solutions you are considering Fix all 130 issues (contact @sf1919 for current list).

Additional context The CoverityMDWorkspaces doc test keeps failing intermittently. The test needs to be more reliable. One potential solution for making the test more reliable is to fix the 130 issues Coverity has identified with CompareMDWorkspaces.cpp.