Angle of \ddots in an uneven matrix

How about this

\documentclass[a4paper]{memoir}
\usepackage{amsmath}
\begin{document}
\[
M = \begin{pmatrix}
    0 & a_{1,2} & a_{1,3}   & \dots     & a_{1,n}   \\
      & 0       & a_{2,3}   & \dots     & a_{2,n}   \\
      &         & 0         & \ddots    & \vdots    \\
      &         &           &     0      & a_{n-1,n}   \\
      &         &           &           & 0
  \end{pmatrix}
\]
\end{document}

BTW: please post small compilable docs instead of sniplets. The less others have to add the more likely it is to get help.