Return reference to TVec in assignement operator

Compiler does not know how to convert fData (i.e. std::vector<T>) to
a TVec<T>& by itself.

math/vecops/inc/ROOT/TVec.hxx

@@ -230,7 +230,7 @@ public:
    // assignment
    TVec<T> &operator=(const TVec<T> &) = default;
    TVec<T> &operator=(TVec<T> &&) = default;
-   TVec<T> &operator=(std::initializer_list<T> ilist) { return fData = ilist; }
+   TVec<T> &operator=(std::initializer_list<T> ilist) { fData = ilist; return *this; }
    // accessors
    reference at(size_type pos) { return fData.at(pos); }
    const_reference at(size_type pos) const { return fData.at(pos); }