Fix from Richard Harke for 64 bit systems. The reftag was left

uninitialized by naNum().  If it happened to be constructing it on the
stack in a location previously occupied by a real reference, it would
generate a corrupt naRef.

simgear/nasal/misc.c

@@ -135,6 +135,7 @@ naRef naNil()
 naRef naNum(double num)
 {
     naRef r;
+    r.ref.reftag = ~NASAL_REFTAG;
     r.num = num;
     return r;
 }