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
diff --git a/fs_mgr/fs_mgr_overlayfs.cpp b/fs_mgr/fs_mgr_overlayfs.cpp
index a7704de..01d7914 100644
--- a/fs_mgr/fs_mgr_overlayfs.cpp
+++ b/fs_mgr/fs_mgr_overlayfs.cpp
@@ -1045,7 +1045,7 @@
 
 static bool CreateScratchOnData(std::string* scratch_device, bool* partition_exists, bool* change) {
     *partition_exists = false;
-    *change = false;
+    if (change) *change = false;
 
     auto images = IImageManager::Open("remount", 10s);
     if (!images) {
@@ -1065,7 +1065,7 @@
         return false;
     }
 
-    *change = true;
+    if (change) *change = true;
 
     // Note: calling RemoveDisabledImages here ensures that we do not race with
     // clean_scratch_files and accidentally try to map an image that will be