jasper: Avoid a deprecation warning

The jas_stream_memopen() function uses any non-negative buffer size with a NULL buffer to signify that the buffer will be allocated internally and set to grow as needed. In the future, jas_stream_memopen() will be replaced by a function that takes a size_t instead of a signed integer.

Passing -1 as the buffer size trips a run time deprecation warning, so we can just use 0 and keep compatibility between the old libjasper and newer versions.

Closes: #73 (closed)