rdopt.c: add an assert before using value in shift

Fixes a clang-19 static analysis warnings of the form:

```
The result of left shift is undefined because the right operand is
negative
```

Bug: 474642915
Change-Id: I410f5119dc29a468b9636f8350fd79aa3b49ed5c

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index fcbc4db..ec3d067 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -4755,6 +4755,7 @@
 
   int skip_motion_mode = 0;
   if (mbmi->partition != PARTITION_NONE) {
+    assert(ref_type > NONE_FRAME);
     int skip_ref = skip_ref_frame_mask & (1 << ref_type);
     if (ref_type <= ALTREF_FRAME && skip_ref) {
       // Since the compound ref modes depends on the motion estimation result of