Fix typo in fixed_matrix move-assignment

The move assignment operator= had an operand of "matrix", which is an
undefined type, causing a compile error.  Changing the type to "fixed_matrix"
solves the problem.

Since this is supposed to be a move-assignment operator, I made it take an
r-value reference as well, which seems to be the general agreed-upon way
to define this operator.

This is related to trac bug #10762