#16920 - MatrixExp insertion in an empty matrix and using colon crashes Scilab.

Fixes issue #16920 (closed)

["a", "a"] will create a matrix of string with a ref equal to 0 (when "a" creates a string with a ref equal to 1 because this is a constant)
In that case, the ["a", "a"] will become the output of the insertion (pOut) and because its ref is at 0, the killMe function will delete it.