garbage collector was not asking for space.

avoid 0 sized calls to mmap.


git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@2058 b08c6af1-5177-4d33-ba66-4b1c6b8b522a

C/heapgc.c

@@ -3867,8 +3867,10 @@ call_gc(UInt gc_lim, Int predarity, CELL *current_env, yamop *nextop)
   /* expand the stack if effectiveness is less than 20 % */
   if (ASP - H < gc_margin/sizeof(CELL) ||
       effectiveness < 20) {
+    UInt sz;
+    
     LeaveGCMode();
-    return Yap_growstack(gc_margin-((ASP-H)*sizeof(CELL)));
+    return Yap_growstack(gc_margin);
   }
   /*
    * debug for(save_total=1; save_total<=N; ++save_total)