Change SHA256 byte counter from size_t to uint64_t

This avoids that the SHA256 implementation would produce wrong paddings
and thus wrong digests for messages of length >= 2^32 bytes on 32-bit
platforms.

This is not exploitable in any way since the SHA256 API is an internal
API and we never call it with that long messages.

src/hash_impl.h

@@ -153,6 +153,8 @@ static void secp256k1_sha256_finalize(secp256k1_sha256 *hash, unsigned char *out
     uint32_t sizedesc[2];
     uint32_t out[8];
     int i = 0;
+    /* The maximum message size of SHA256 is 2^64-1 bits. */
+    VERIFY_CHECK(hash->bytes < ((uint64_t)1 << 61));
     sizedesc[0] = BE32(hash->bytes >> 29);
     sizedesc[1] = BE32(hash->bytes << 3);
     secp256k1_sha256_write(hash, pad, 1 + ((119 - (hash->bytes % 64)) % 64));