Flonum unboxing too agressive

[sjamaan]

Found by Sven Hartrumpf. The following program will segfault:

(define (test-it)
  (let loop ((done #f))
    (if done
        '()
        (cons  (or (read) 0.0) (loop #t)))))

(print (test-it))

When run with a DEBUGBUILD CHICKEN, you'll see it fails with a nice panic due to a low-level assertion error triggered by a C_flonum_magnitude call when you enter a fixnum:

[panic] Low-level type assertion C_blockp((C_word)C_VAL1(C__PREV_TMPST.n1))=#t failed at test.c:176 - execution terminated

Generated code failing:

/* k150 in loop in test-it in k131 in k128 in k125 */
static void C_ccall f_152(C_word c,C_word *av){
C_word tmp;
C_word t0=av[0];
C_word t1=av[1];
C_word t2;
C_word t3;
C_word t4;
double f0;
C_word *a;
C_check_for_interrupt;
if(C_unlikely(!C_demand(C_calculate_demand(8,c,2)))){
C_save_and_reclaim((void *)f_152,2,av);}
a=C_alloc(8);
f0=C_flonum_magnitude((C_truep(t1)?t1:lf[1])); /// <--- offending code
t2=C_flonum(&a,f0);
t3=(*a=C_CLOSURE_TYPE|3,a[1]=(C_word)f_159,a[2]=((C_word*)t0)[2],a[3]=t2,tmp=(C_word)a,a+=4,tmp);
C_trace(C_text("test.scm:5: loop"));
t4=((C_word*)((C_word*)t0)[3])[1];
f_141(t4,t3,C_SCHEME_TRUE);}

Most likely introduced by 79cf7427638eebf695f2be54731bb6bbf4d0fff2.