Fix "pointer is null" static analyzer warning. NFCI.

Remove Ctx null test as clang static analyzer assumes that this can fail - replace it with an assertion as the pointer is always dereferenced below.

Author: RKSimon

clang/lib/Sema/SemaTemplate.cpp

@@ -7386,7 +7386,8 @@ Sema::CheckTemplateDeclScope(Scope *S, TemplateParameterList *TemplateParams) {
   // C++ [temp]p4:
   //   A template [...] shall not have C linkage.
   DeclContext *Ctx = S->getEntity();
-  if (Ctx && Ctx->isExternCContext()) {
+  assert(Ctx && "Unknown context");
+  if (Ctx->isExternCContext()) {
     Diag(TemplateParams->getTemplateLoc(), diag::err_template_linkage)
         << TemplateParams->getSourceRange();
     if (const LinkageSpecDecl *LSD = Ctx->getExternCContext())