refactor: Avoid UB in SHA3_256::Write

It is UB to apply a distance to a pointer or iterator further than the
end itself, even if the distance is (partially) revoked later on.

Fix the issue by advancing the data pointer at most to the end.

src/crypto/sha3.cpp

@@ -105,9 +105,9 @@ void KeccakF(uint64_t (&st)[25])
 
 SHA3_256& SHA3_256::Write(Span<const unsigned char> data)
 {
-    if (m_bufsize && m_bufsize + data.size() >= sizeof(m_buffer)) {
+    if (m_bufsize && data.size() >= sizeof(m_buffer) - m_bufsize) {
         // Fill the buffer and process it.
-        std::copy(data.begin(), data.begin() + sizeof(m_buffer) - m_bufsize, m_buffer + m_bufsize);
+        std::copy(data.begin(), data.begin() + (sizeof(m_buffer) - m_bufsize), m_buffer + m_bufsize);
         data = data.subspan(sizeof(m_buffer) - m_bufsize);
         m_state[m_pos++] ^= ReadLE64(m_buffer);
         m_bufsize = 0;