snapuserd: fix a use of an uninitialized value

`header` is only initialized in the `if` block of this condition. Hence,
its use in the `else` portion isn't correct. Refactor the code a bit to
make this kind of bug a bit harder to write in the future.

Caught by the static analyzer:
> system/core/fs_mgr/libsnapshot/snapuserd.cpp:457:9: warning: 1st
function call argument is an uninitialized value
[clang-analyzer-core.CallAndMessage]

Bug: None
Test: TreeHugger

Change-Id: Ie56578520acf3cc972efa3336e40698feed20200
diff --git a/fs_mgr/libsnapshot/snapuserd.cpp b/fs_mgr/libsnapshot/snapuserd.cpp
index d620300..4c4a342 100644
--- a/fs_mgr/libsnapshot/snapuserd.cpp
+++ b/fs_mgr/libsnapshot/snapuserd.cpp
@@ -447,16 +447,15 @@
 }
 
 void Snapuserd::CheckMergeCompletionStatus() {
-    CowHeader header;
-
-    if (merge_initiated_) {
-        reader_->GetHeader(&header);
-        SNAP_LOG(INFO) << "Merge-status: Total-Merged-ops: " << header.num_merge_ops
-                       << " Total-data-ops: " << reader_->total_data_ops();
-    } else {
-        SNAP_LOG(INFO) << "Merge was not initiated. Total-Merged-ops: " << header.num_merge_ops
-                       << " Total-data-ops: " << reader_->total_data_ops();
+    if (!merge_initiated_) {
+        SNAP_LOG(INFO) << "Merge was not initiated. Total-data-ops: " << reader_->total_data_ops();
+        return;
     }
+
+    CowHeader header;
+    reader_->GetHeader(&header);
+    SNAP_LOG(INFO) << "Merge-status: Total-Merged-ops: " << header.num_merge_ops
+                   << " Total-data-ops: " << reader_->total_data_ops();
 }
 
 /*