bundle/commands/exec: fix exit code handling.

In some `brew tests` runs, `$CHILD_STATUS` can be `nil`.
Let's handle this and avoid these flaky tests.

Library/Homebrew/bundle/commands/exec.rb

@@ -178,7 +178,9 @@ module Homebrew
             exit_code = 0
             run_services(@dsl.entries) do
               Kernel.system(*args)
-              exit_code = $CHILD_STATUS.exitstatus
+              if (system_exit_code = $CHILD_STATUS&.exitstatus)
+                exit_code = system_exit_code
+              end
             end
             exit!(exit_code)
           else