Restore null byte in to_ptr for FFI backend (#187)

Addresses #180 (comment)

Author: ankane

lib/fiddle/ffi_backend.rb

@@ -241,9 +241,7 @@ def self.from_native(value, ctx)
 
     def self.to_ptr(value)
       if value.is_a?(String)
-        cptr = Pointer.malloc(value.bytesize)
-        cptr.ffi_ptr.put_bytes(0, value)
-        cptr
+        Pointer.new(FFI::MemoryPointer.from_string(value), value.bytesize)
 
       elsif value.is_a?(Array)
         raise NotImplementedError, "array ptr"

test/fiddle/test_pointer.rb

@@ -134,6 +134,8 @@ def test_to_ptr_string
       ptr = Pointer[str]
       assert_equal str.length, ptr.size
       assert_equal 'hello', ptr[0,5]
+      assert_equal 0, Fiddle::Pointer.to_ptr("abc")[3]
+      assert_equal "world", Fiddle::Pointer.to_ptr("hello\0world")[6,5]
     end
 
     def test_to_ptr_io