Let s48-do-gc return 0 so the PreScheme compiler will emit the correct

signature of the function.

scheme/vm/gc.scm

@@ -105,7 +105,9 @@
 	     (error "GC error: ran out of space in new heap"))
 	    ((address< end (heap-pointer))
 	     (loop end)))))
-  (clean-weak-pointers))
+  (clean-weak-pointers)
+  0);; for the PreScheme compiler which otherwise will emit return statements 
+    ;; but declare the function to have return type void
 
 (define (s48-trace-stob-contents! stob)
   (let ((start (address-after-header stob))