Correctly handle wrap around in ring buffer used internally to buffer application data

Motivation:

We didn't correctly handle the case of the ring buffer wraping around and so could up casting an negative value to size_t which would result in producting a value close to SIZE_MAX that then would be used to memcpy.
This had the result of a heap buffer overflow. The likelyness of this increased if the nonApplicationBufferSize was set to a very low value.

Modifications:

Correctly use modulo to calculate the startIndex.

Result:

No more overflow possible

Author: normanmaurer

openssl-dynamic/src/main/c/ssl.c

@@ -196,7 +196,9 @@ static jint tcn_write_to_bytebuffer(BIO* bio, const char* in, int inl) {
         }
 
         writeAmount = TCN_MIN(nonApplicationBufferFreeSpace, (jint) inl) * sizeof(char);
-        startIndex = bioUserData->nonApplicationBufferOffset + bioUserData->nonApplicationBufferLength;
+        // use modulo to account for wrap around the ring buffer that buffers data.
+        startIndex = (bioUserData->nonApplicationBufferOffset + bioUserData->nonApplicationBufferLength)
+                                          % bioUserData->nonApplicationBufferSize;
         writeChunk = bioUserData->nonApplicationBufferSize - startIndex;
 
 #ifdef NETTY_TCNATIVE_BIO_DEBUG