fixup portage provider to be more robust about package names

Signed-off-by: Opscode, Inc. <oss@opscode.com>

Author: Ezra Zygmuntowicz

chef/lib/chef/provider/package/portage.rb

@@ -56,7 +56,7 @@ def parse_emerge(package, txt)
             if line =~ /\*(.*)/
               pkg = $1.strip
             end
-            if pkg == package or pkg.split('/').last == package
+            if (pkg == package) || (pkg.split('/').last == package rescue false)
               if line =~ /Latest version available: (.*)/
                 available = $1
               elsif line =~ /Latest version installed: (.*)/