There are reported crashes on Android that suggest a null
"tables" attribute in DownloadDCThread. This should never
happen, as the table() function connected to this attribute
returns the address of a subobject. Thus, even if the original
DownloadThread is null, this would not return a null pointer
(the subobject is not at address 0).
Catch these null-object accesses and write a warning message
to the console. Hopefully, this will help is localizing the
problem.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>