io.c (io_write_nonblock): add RB_GC_GUARD, io_fflush may switch threads

Since io_fflush may block on mutex or rb_io_wait_readable and
switch threads, we need to ensure the `str' VALUE returned by
`rb_obj_as_string` is visible to GC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66242 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

io.c

@@ -3001,6 +3001,7 @@ io_write_nonblock(VALUE io, VALUE str, VALUE ex)
 
     rb_io_set_nonblock(fptr);
     n = write(fptr->fd, RSTRING_PTR(str), RSTRING_LEN(str));
+    RB_GC_GUARD(str);
 
     if (n < 0) {
 	int e = errno;