Handle CXXDeductionGuide in ReSubstTemplateArg.

This fixes ROOT-9147.

Also avoid future cases of nullptr dereference

Author: pcanal

core/clingutils/src/TClingUtils.cxx

@@ -4585,11 +4585,13 @@ clang::QualType ROOT::TMetaUtils::ReSubstTemplateArg(clang::QualType input, cons
          } else {
             replacedCtxt = decl->getDescribedClassTemplate();
          }
+      } else if (auto const declguide = llvm::dyn_cast<clang::CXXDeductionGuideDecl>(replacedDeclCtxt)) {
+         replacedCtxt = llvm::dyn_cast<clang::ClassTemplateDecl>(declguide->getDeducedTemplate());
       } else {
          replacedCtxt = llvm::dyn_cast<clang::ClassTemplateDecl>(replacedDeclCtxt);
       }
 
-      if (replacedCtxt->getCanonicalDecl() == TSTdecl->getSpecializedTemplate()->getCanonicalDecl()
+      if ((replacedCtxt && replacedCtxt->getCanonicalDecl() == TSTdecl->getSpecializedTemplate()->getCanonicalDecl())
           || /* the following is likely just redundant */
           substType->getReplacedParameter()->getDecl()
           == TSTdecl->getSpecializedTemplate ()->getTemplateParameters()->getParam(index))