Dependency.expand: ensure pop stack

During the dependencies expansion, there may be errors (e.g. FormulaUnavaiableError).
As result, some deps will be left behind in the stack and interfere afterwards
dependencies expansion.

So let's ensure stack clean for each expansions.

Fixes Homebrew/homebrew#48834.

Library/Homebrew/dependency.rb

@@ -95,8 +95,9 @@ class Dependency
         end
       end
 
-      @expand_stack.pop
       merge_repeats(expanded_deps)
+    ensure
+      @expand_stack.pop
     end
 
     def action(dependent, dep, &_block)