Fix CVE-2019-15845

* https://www.ruby-lang.org/en/news/2019/10/01/nul-injection-file-fnmatch-cve-2019-15845/
* ruby/ruby@a0a2640

Author: eregon

src/main/ruby/truffleruby/core/file.rb

@@ -719,6 +719,7 @@ def self.braces(pattern, flags=0, patterns=[])
 
   def self.fnmatch(pattern, path, flags=0)
     pattern = StringValue(pattern)
+    Truffle::Type.check_null_safe(pattern)
     path    = Truffle::Type.coerce_to_path(path)
     flags   = Truffle::Type.coerce_to_int(flags)
     brace_match = false