Fix Coverity defects in soft-pkcs11 test code

Fix Coverity defects in soft-pkcs11 test code

Nothing in the code removes objects from soft_token.object.obs, so
simplify add_st_object() not to search for an empty slot.  Avoid using
random() by using a counter for session handles and just the array
slot number for object handles.  Add a helper get_rcfilename() to
facilitate checking the result of asprintf().  Properly initialize ap
in sprintf_fill().  Close the file handle in read_conf_file().