Handle CXXDeductionGuide in ReSubstTemplateArg.
Handle CXXDeductionGuide in ReSubstTemplateArg.

This fixes ROOT-9147.

Also avoid future cases of nullptr dereference

karies/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,root-mirror/root,karies/root,olifre/root,olifre/root,olifre/root,karies/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root