bottle: avoid raising exception in ensure block

The `ensure` block can be reached before the tab is modified (and thus
before `original_tab` gets populated). Avoid raising an exception when
attempting to restore a tab that hasn't been modified yet, e.g. because
relocating some of the Mach-O binaries failed.

Library/Homebrew/cmd/bottle.rb

@@ -255,7 +255,7 @@ module Homebrew
         raise
       ensure
         ignore_interrupts do
-          original_tab.write
+          original_tab.write if original_tab
           keg.relocate_install_names Keg::PREFIX_PLACEHOLDER, prefix,
             Keg::CELLAR_PLACEHOLDER, cellar
           keg.relocate_text_files Keg::PREFIX_PLACEHOLDER, prefix,