envutil.rb: backtrace may not be present

* test/ruby/envutil.rb (assert_separately): SystemStackError at
  machine stack overflow on platforms where sigaltstack is
  unavailable does not have backtrace.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44130 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/envutil.rb

@@ -345,8 +345,10 @@ eom
           ignore_stderr = nil
         end
         if res
-          res.backtrace.each do |l|
-            l.sub!(/\A-:(\d+)/){"#{file}:#{line + $1.to_i}"}
+          if bt = res.backtrace
+            bt.each do |l|
+              l.sub!(/\A-:(\d+)/){"#{file}:#{line + $1.to_i}"}
+            end
           end
           raise res
         end