Fix memory access errors in CPU build.

[pelesh]

Description

Fixed following issues:

• Vector destructor logic incorrect, tries to delete data that has not been allocated.
• KLU test allocates and moves data to GPU without need to do so. Causes a segfault when Re::Solve is built without GPU support.

Closes #286

Proposed changes

• Vector destructor deletes only if data exists (pointer is not null) and vector owns that data.
• Remove commands moving data to GPU from KLU test.

Checklist

• All tests pass. Code tested on
  • CPU backend
  • CUDA backend
  • HIP backend
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows Re::Solve style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining
why you chose the solution you did and what alternatives you considered, etc.

[shakedregev]

Tests and examples work. Merging