Fix SEGV at eof? on null StringIO

Author: nobu

ext/java/org/jruby/ext/stringio/StringIO.java

@@ -650,7 +650,7 @@ public IRubyObject each_char(final ThreadContext context, final Block block) {
     public IRubyObject eof(ThreadContext context) {
         checkReadable();
         StringIOData ptr = getPtr();
-        if (ptr.pos < ptr.string.size()) return context.fals;
+        if (!isEndOfString()) return context.fals;
         return context.tru;
     }
 

test/stringio/test_stringio.rb

@@ -70,6 +70,11 @@ def test_null
     assert_nil io.getc
   end
 
+  def test_pread_eof
+    io = StringIO.new(nil)
+    assert_predicate io, :eof?
+  end
+
   def test_pread_null
     io = StringIO.new(nil)
     assert_raise(EOFError) { io.pread(1, 0) }