When trying the GDAL OpenJPEG driver against openjpeg current master HEAD,
I get failures when trying to create .jp2 files. The driver uses
opj_write_tile() and in some tests numresolutions = 1.

In openjp2/dwt.c:410, l_data_size = opj_dwt_max_resolution( tilec->resolutions,tilec->numresolutions) * (OPJ_UINT32)sizeof(OPJ_INT32);
is called and returns l_data_size = 0. Now in git opj_malloc() has a special case
for 0 to return a NULL pointer whereas previously it relied on system malloc(),
which in my case didn't return NULL.

So only test the pointer value if l_data_size != 0. This makes the GDAL
autotest suite to pass again.