Add null check for partial in DoUpdateCachedResponseComplete

Bug: 780029
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I4130623a8e72803385189f8ce7e2d50bb998ad48
Reviewed-on: https://chromium-review.googlesource.com/746536Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: Shivani Sharma <shivanisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512875}

net/http/http_cache_transaction.cc

@@ -1796,7 +1796,7 @@ int HttpCache::Transaction::DoUpdateCachedResponseComplete(int result) {
   } else if (entry_ && !handling_206_) {
     DCHECK_EQ(READ_WRITE, mode_);
     if ((!partial_ && !cache_->IsWritingInProgress(entry_)) ||
-        partial_->IsLastRange()) {
+        (partial_ && partial_->IsLastRange())) {
       mode_ = READ;
     }
     // We no longer need the network transaction, so destroy it.