Clean up child process handles.

This matters now that we use a control socket.

US-461 #done

Change-Id: Icd557ce370ea72595cf4061894c847c4117ab2cc
diff --git a/fuchsia/fuchsia-compat.c b/fuchsia/fuchsia-compat.c
index d2862a8..3ca14a6 100644
--- a/fuchsia/fuchsia-compat.c
+++ b/fuchsia/fuchsia-compat.c
@@ -99,6 +99,9 @@
     handler(SIGCHLD);
   }
 
+  zx_handle_close(child->handle);
+  child->handle = ZX_HANDLE_INVALID;
+
   return NULL;
 }