Avoid out-of-bound pointers and integer overflows in size comparisons

This changes pointer calculations in size comparions to a form that
ensures that no out-of-bound pointers are computed, because even their
computation yields undefined behavior.
Also, this changes size comparions to a form that ensures that neither
the left-hand side nor the right-hand side can overflow.

src/hash_impl.h

@@ -131,7 +131,8 @@ static void secp256k1_sha256_transform(uint32_t* s, const uint32_t* chunk) {
 static void secp256k1_sha256_write(secp256k1_sha256 *hash, const unsigned char *data, size_t len) {
     size_t bufsize = hash->bytes & 0x3F;
     hash->bytes += len;
-    while (bufsize + len >= 64) {
+    VERIFY_CHECK(hash->bytes >= len);
+    while (len >= 64 - bufsize) {
         /* Fill the buffer, and process it. */
         size_t chunk_len = 64 - bufsize;
         memcpy(((unsigned char*)hash->buf) + bufsize, data, chunk_len);