Don't call methods on xcrun_path if it's nil

Library/Homebrew/macos.rb

@@ -36,12 +36,11 @@ module MacOS extend self
         # look in dev_tools_path, and finally in xctoolchain_path, because the
         # tools were split over two locations beginning with Xcode 4.3+.
         xcrun_path = unless Xcode.bad_xcode_select_path?
-          `/usr/bin/xcrun -find #{tool} 2>/dev/null`.chomp
+          path = `/usr/bin/xcrun -find #{tool} 2>/dev/null`.chomp
+          # If xcrun finds a superenv tool then discard the result.
+          path unless path.include?(HOMEBREW_PREFIX/"Library/ENV")
         end
 
-        # If xcrun finds a superenv tool then discard the result.
-        xcrun_path = nil if xcrun_path.include? HOMEBREW_PREFIX+"Library/ENV"
-
         paths = %W[#{xcrun_path}
                    #{dev_tools_path}/#{tool}
                    #{xctoolchain_path}/usr/bin/#{tool}]