Fix a pulley crash on OSS-Fuzz (#9042)

This fixes a crash where the `GetSp` opcode was overwriting a special
register, so apply a similar filter as to other instructions to ensure
that the special registers are not clobbered.

pulley/fuzz/src/interp.rs

@@ -107,6 +107,6 @@ fn extended_op_is_safe_for_fuzzing(op: &ExtendedOp) -> bool {
     match op {
         ExtendedOp::Trap(_) => true,
         ExtendedOp::Nop(_) => true,
-        ExtendedOp::GetSp(_) => true,
+        ExtendedOp::GetSp(GetSp { dst, .. }) => !dst.is_special(),
     }
 }