aix: fix un-initialized pointer field in fs handle

In AIX, fs watch close call was corrupting memory in the compiler.
The handle->dir_filename field can be un-initialized, if the watch
is initiated but not event got fired. But the uv_fs_event_stop was
freeing this pointer as if it was malloc'ed, leading to the crash.

Properly initialize handle-dir_filename to avoid a garbage pointer.
Fixes: nodejs#13577

Author: gireeshpunathil

deps/uv/src/unix/aix.c

@@ -855,6 +855,7 @@ int uv_fs_event_start(uv_fs_event_t* handle,
   uv__io_init(&handle->event_watcher, uv__ahafs_event, fd);
   handle->path = uv__strdup(filename);
   handle->cb = cb;
+  handle->dir_filename = NULL;
 
   uv__io_start(handle->loop, &handle->event_watcher, POLLIN);