Use mrb_to_flo() to check return value from to_f; fix #4115

The ISO standard does not include implicit type conversion using
`to_int`, `to_str` and sometimes `to_f`. For the compactness of the
mruby implementation, maybe we should remove those implicit conversion
from mruby.

mrbgems/mruby-pack/src/pack.c

@@ -1127,7 +1127,8 @@ mrb_pack_pack(mrb_state *mrb, mrb_value ary)
 #ifndef MRB_WITHOUT_FLOAT
       } else if (type == PACK_TYPE_FLOAT) {
         if (!mrb_float_p(o)) {
-          o = mrb_funcall(mrb, o, "to_f", 0);
+          mrb_float f = mrb_to_flo(mrb, o);
+          o = mrb_float_value(mrb, f);
         }
 #endif
       } else if (type == PACK_TYPE_STRING) {