(compare_files): fix -Wstringop-overread warning exposed on s390x…

… (GCC 11)

```
../../../src/filemanager/cmd.c: In function ‘compare_files’:
../../../src/filemanager/cmd.c:197:37: warning: ‘memcmp’ specified bound 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Wstringop-overread]
  197 |             result = (n1 != n2) || (memcmp (buf1, buf2, n1) != 0);
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~
```

Signed-off-by: Yury V. Zaytsev <[email protected]>

src/filemanager/cmd.c

@@ -195,7 +195,7 @@ compare_files (const vfs_path_t *vpath1, const vfs_path_t *vpath2, off_t size)
                     ;
             }
             while (n1 == n2 && n1 == sizeof (buf1) && memcmp (buf1, buf2, sizeof (buf1)) == 0);
-            result = (n1 != n2) || (memcmp (buf1, buf2, n1) != 0);
+            result = (n1 != n2) || (memcmp (buf1, buf2, MIN (n1, sizeof (buf1))) != 0);
             rotate_dash (FALSE);
 
             close (file2);