dpnet/tests: Avoid casting return value of HeapAlloc calls

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=15042

Your paranoid android.


=== WNT4WSSP6 (32 bit peer) ===
peer.c:52: Test failed: CoCreateInstance failed with 0x80040154
peer: unhandled exception c0000005 at 004013BF

=== W2KPROSP4 (32 bit peer) ===
peer.c:52: Test failed: CoCreateInstance failed with 0x80040154
peer: unhandled exception c0000005 at 004013BF