fs_mgr: fix potential uses of nullptr

Clang's static analyzer flagged the following potential null pointer
dereferences. Looks like a transitive caller of this function has

```
if (change) *change = foo;
```

...and no nullness checks between that and these unconditional
assignments to `*change`.

> system/core/fs_mgr/fs_mgr_overlayfs.cpp:1100:13: warning: Dereference
of null pointer (loaded from variable 'change')
[clang-analyzer-core.NullDereference] >
system/core/fs_mgr/fs_mgr_overlayfs.cpp:1167:17: warning: Dereference of
null pointer (loaded from variable 'change')
[clang-analyzer-core.NullDereference]

Bug: None
Test: TreeHugger
Change-Id: I656e3b0f56b18ec3ca3d1db773feb81adf163122