fix memory leak in allocExec/freeExec (see bug #985)

rts/sm/Storage.c

@@ -1046,13 +1046,17 @@ void freeExec (void *addr)
     bd->gen_no -= *(StgPtr)p;
     *(StgPtr)p = 0;
 
+    if (bd->gen_no == 0) {
         // Free the block if it is empty, but not if it is the block at
         // the head of the queue.
-    if (bd->gen_no == 0 && bd != exec_block) {
+        if (bd != exec_block) {
             debugTrace(DEBUG_gc, "free exec block %p", bd->start);
             dbl_link_remove(bd, &exec_block);
             setExecutable(bd->start, bd->blocks * BLOCK_SIZE, rtsFalse);
             freeGroup(bd);
+        } else {
+            bd->free = bd->start;
+        }
     }
 
     RELEASE_SM_LOCK