fix(crypto): adjust random_permute

What changed, and why it matters

This commit fixes a function that shuffles arrays (like recovery-seed word order or on-screen PIN layouts) in Trezor's cryptographic code. The original code could misbehave or even crash when given very short or very long inputs, and it used a signed integer type that could turn negative in unexpected ways. The patch adds a safety check, skips shuffling when there is nothing to shuffle, and uses the correct unsigned type. The practical security impact is moderate: it removes a source of non-random or buggy shuffling that could leak information or cause crashes, but the commit itself does not claim to fix an active exploit.

Security signals we found

Evidence from the diff

Changed components

### crypto/rand.c
@@ -21,6 +21,8 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include <assert.h>
+
 #include "rand.h"
 
 uint32_t random_uniform(uint32_t n) {
@@ -30,7 +32,11 @@ uint32_t random_uniform(uint32_t n) {
 }
 
 void random_permute(char *str, size_t len) {
-  for (int i = len - 1; i >= 1; i--) {
+  assert(len <= UINT32_MAX);
+  if (len < 2) {
+    return;
+  }
+  for (size_t i = len - 1; i >= 1; i--) {
     uint32_t j = random_uniform(i + 1);
     char t = str[j];
     str[j] = str[i];