[Clang][NFC] Add nullptr check in InitializationSequence::InitializeFrom (#143067)

Static analysis flagged that Var could be nullptr but we were not
checking in the branch and unconditionally dereferences the pointer.

Note, code was added by 576161c

Author: shafik

clang/lib/Sema/SemaInit.cpp

@@ -6620,7 +6620,7 @@ void InitializationSequence::InitializeFrom(Sema &S,
       // initializer present. However, we only do this for structure types, not
       // union types, because an unitialized field in a union is generally
       // reasonable, especially in C where unions can be used for type punning.
-      if (!Initializer && !Rec->isUnion() && !Rec->isInvalidDecl()) {
+      if (Var && !Initializer && !Rec->isUnion() && !Rec->isInvalidDecl()) {
         if (const FieldDecl *FD = getConstField(Rec)) {
           unsigned DiagID = diag::warn_default_init_const_field_unsafe;
           if (Var->getStorageDuration() == SD_Static ||