io-jasper.c: Validate jas_matrix_create result
io-jasper.c: Validate jas_matrix_create result

jas_matrix_create can return NULL when memory allocation
fails.

Closes: #107

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf