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