Check if mount directory exists before attempting mount() syscall

Test: treehugger
Bug: 172696594
Change-Id: If5d0971a5443db2f26cddb31ac2034112e346fe2
diff --git a/payload_consumer/postinstall_runner_action.cc b/payload_consumer/postinstall_runner_action.cc
index bd49639..37a8577 100644
--- a/payload_consumer/postinstall_runner_action.cc
+++ b/payload_consumer/postinstall_runner_action.cc
@@ -143,6 +143,11 @@
   fs_mount_dir_ = temp_dir.value();
 #endif  // __ANDROID__
 
+  if (!utils::FileExists(fs_mount_dir_.c_str())) {
+    LOG(ERROR) << "Mount point " << fs_mount_dir_
+               << " does not exist, mount call will fail";
+    return CompletePostinstall(ErrorCode::kPostinstallRunnerError);
+  }
   // Double check that the fs_mount_dir is not busy with a previous mounted
   // filesystem from a previous crashed postinstall step.
   if (utils::IsMountpoint(fs_mount_dir_)) {