Ensure that the text field is terminated when writing EBU STL files

The text field must be terminated with 0x8F, so write an extra block if
the last block is filled exactly.

Originally committed to SVN as r6637.

aegisub/src/subtitle_format_ebu3264.cpp

@@ -546,9 +546,14 @@ namespace
 
 				tti.push_back(base);
 				// write an extension block number if the remaining text doesn't fit in the block
-				tti.back().ebn = bytes_remaining > block_size ? num_blocks++ : 255;
+				tti.back().ebn = bytes_remaining >= block_size ? num_blocks++ : 255;
 
 				std::copy(&fullstring[pos], &fullstring[pos + std::min(block_size, bytes_remaining)], tti.back().tf);
+
+				// Write another block for the terminator if we exactly used up
+				// the last block
+				if (bytes_remaining == block_size)
+					tti.push_back(base);
 			}
 		}