libstore: fix UAF in FileTransfer

after startTransfer the transfer, and thus downloadState, is gone. asan
hasn't caught this, presumably because the access is in libc somewhere.
we don't even need access to the old state; the assertion is not useful
here and clearing the previous exception is invisible to the new round.

Change-Id: I32dc1a487b96cbaabcf5061c9b1f96dbc5aaae49

lix/libstore/filetransfer.cc

@@ -845,10 +845,6 @@ struct curlFileTransfer : public FileTransfer
 
         bool attemptRetry(const std::string & context)
         {
-            auto state(transfer->downloadState.lock());
-
-            assert(state->data.empty());
-
             thread_local std::minstd_rand random{std::random_device{}()};
             std::uniform_real_distribution<> dist(0.0, 0.5);
             int ms = parent.baseRetryTimeMs * std::pow(2.0f, attempt - 1 + dist(random));
@@ -860,8 +856,6 @@ struct curlFileTransfer : public FileTransfer
 
             std::this_thread::sleep_for(std::chrono::milliseconds(ms));
 
-            state->exc = nullptr;
-
             // use the effective URI of the previous transfer for retries. this avoids
             // some silent corruption if a redirect changes between starting and retry
             const auto & uri = metadata.effectiveUri.empty() ? this->uri : metadata.effectiveUri;