Saturate the reference POC LSB after a memory management op 5

A saturated top POC with a negative bottom field could still overflow
here, as CodeRabbit noted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/video/src/h264/slice.rs

@@ -532,7 +532,7 @@ impl PocState {
         if sh.nal_ref_idc != 0 {
             if mmco5 {
                 self.prev_ref_msb = 0;
-                self.prev_ref_lsb = poc.top - poc.frame();
+                self.prev_ref_lsb = poc.top.saturating_sub(poc.frame());
             } else {
                 self.prev_ref_msb = poc.msb;
                 self.prev_ref_lsb = sh.pic_order_cnt_lsb as i32;