Don't clear fs_mount_dir_

During postinstall, fs_mount_dir_ is cleared after postinstall is
completed for a partition. If there are more than 1 partition which runs
postinstall, the next partition will see an empty fs_mount_dir_ and
fail.

Test: treehugger
Bug: 175232022

Change-Id: I42110d0e2a886b631f0984edcdc7a916b1121e3e
diff --git a/payload_consumer/postinstall_runner_action.cc b/payload_consumer/postinstall_runner_action.cc
index 6a6d687..dc444c7 100644
--- a/payload_consumer/postinstall_runner_action.cc
+++ b/payload_consumer/postinstall_runner_action.cc
@@ -63,6 +63,8 @@
   TEST_AND_RETURN(base::CreateNewTempDirectory("au_postint_mount", &temp_dir));
   fs_mount_dir_ = temp_dir.value();
 #endif  // __ANDROID__
+  CHECK(!fs_mount_dir_.empty());
+  LOG(INFO) << "postinstall mount point: " << fs_mount_dir_;
 }
 
 void PostinstallRunnerAction::PerformAction() {
@@ -315,7 +317,6 @@
     PLOG(WARNING) << "Not removing temporary mountpoint " << fs_mount_dir_;
   }
 #endif
-  fs_mount_dir_.clear();
 
   progress_fd_ = -1;
   progress_controller_.reset();