skeletonization: fix crash with low number of proxies

Managed to hit a case where the `src_mat` had shape `(21, 23)`
and the `tgt_mat` had shape `(23, 22)`. The rank after skeletonizing
the target was `22`, which was smaller then the max one for the source
and caused `interp_decomp` to crash somewhere in the Fortran code.