Duplicate Heap structure on execve if it's shared

When parent uses vfork() and then child performs execve()
we previously just cleared heap struct which was shared
in case of vfork() and it remained shared between parent and child

Now during execve when Heap structure is shared we create new one
and unlink child from parent Heap

src/execve/exit.c

@@ -465,7 +465,14 @@ void translate_execve_exit(Tracee *tracee)
 	}
 
 	/* New processes have no heap.  */
-	bzero(tracee->heap, sizeof(Heap));
+	if (talloc_reference_count(tracee->heap) >= 1) {
+		talloc_unlink(tracee, tracee->heap);
+		tracee->heap = talloc_zero(tracee, Heap);
+		if (tracee->heap == NULL)
+			note(tracee, ERROR, INTERNAL, "can't alloc heap after execve");
+	} else {
+		bzero(tracee->heap, sizeof(Heap));
+	}
 
 	/* Transfer the load script to the loader.  */
 	status = transfer_load_script(tracee);