Fix read of uninitialized threshold in SparseQR

Reference issue

none

What does this implement/fix?

This fixes a read of uninitialized data m_threshold in SparseQR::factorize(). Currently, this uninitialized data is overwritten in the same function and thus not a bug. Not reading it in the first place is a cleaner approach though.

Additional information

This issue was reported by Coverity scan.