lib/drb/extservm.rb (service): do not return `false'

invoke_service_command may set entries in @servers to `false',
making it incompatible with the intended use of the
safe navigation operator.

This caused occasional DRb test failures, but they were hidden
with automatic retry.

[ruby-core:87524] [Bug #14856]

Fixes: r53111 ("use safe navigation operator")
	commit 059c9c1cf3 [GH-1142]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63698 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/drb/extservm.rb

@@ -37,7 +37,7 @@ module DRb
       synchronize do
         while true
           server = @servers[name]
-          return server if server&.alive?
+          return server if server && server.alive? # server may be `false'
           invoke_service(name)
           @cond.wait
         end