fix closure GC handling when closure is only half-formed

src/racket/src/mzclpf_post.inc

@@ -10,6 +10,8 @@
   if (data) {
     /* GLOBAL ASSUMPTION: prefix is at the end of a closure */
     Scheme_Prefix *pf = (Scheme_Prefix *)c->vals[closure_size - 1];
+    
+    if (pf) {
       /* Since pf hasn't been marked, we don't need a GC_resolve(): */
       int *use_bits = PREFIX_TO_USE_BITS(pf);
       uintptr_t map;
@@ -62,3 +64,4 @@
         }
       }
     }
+  }