RingBuffer: Limit readblocksize based upon the default buffer size.

Now that the frontend RingBuffer can buffer up to 32Mb, large read block
sizes lead to read failures in the backend as the block size can exceed
the size of the default buffer size.

mythtv/libs/libmythtv/ringbuffer.cpp

@@ -765,7 +765,7 @@ void RingBuffer::run(void)
                     (now.tv_usec - lastread.tv_usec) / 1000;
                 readtimeavg = (readtimeavg * 9 + readinterval) / 10;
 
-                if (readtimeavg < 150 && (uint)readblocksize < (bufferSize>>2))
+                if (readtimeavg < 150 && (uint)readblocksize < (BUFFER_SIZE_MINIMUM >>2))
                 {
                     int old_block_size = readblocksize;
                     readblocksize = 3 * readblocksize / 2;