make-string et al nulls memory if not given an initializer
* libguile/gc-malloc.c: Add a note that the gc-malloc does not clear the
memory block, so users need to make sure it is initialized.
* libguile/bitvectors.c (scm_c_make_bitvector):
* libguile/bytevectors.c (scm_make_bytevector):
* libguile/strings.c (scm_c_make_string): If no initializer is given,
initialize the bytes to 0. Prevents information leakage if an app uses
make-string et al without initializers.
* libguile/foreign.c (make_cif): Initialize this too, to prevent leakage
in the struct holes. Paranoia...