Fix for error: writing 8 bytes into a region of size 4

[faveoled]

The current code fails to compile on Ubuntu 24.04:

rsa.c: In function ‘RSA_verify’:
rsa.c:283:16: error: writing 8 bytes into a region of size 4 [-Werror=stringop-overflow=]
  283 |         buf[i] ^= *hash++;
      |                ^~

This PR isn't a proper fix but a way to get this issue more attention.