MB-34410: [SR] Warmup::loadPreparedSyncWrites: skip missing vBuckets

In loadPreparedSyncWrites() the result of getVBucket(vbid) is not
checked to see if it's a non-null ptr, unlike other warmup phases. As
such, if ns_server deletes a vBucket before loadPreparedSyncWrites,
then we can crash dereferencig a null ptr.

Change-Id: I6e0639716d35a2e0f4db92d8b2cdfbfb09c9f5c5
Reviewed-on: http://review.couchbase.org/109972
Reviewed-by: Jim Walker <[email protected]>
Tested-by: Dave Rigby <[email protected]>

Author: daverigby

engines/ep/src/warmup.cc

@@ -1168,6 +1168,9 @@ void Warmup::loadPreparedSyncWrites(uint16_t shardId) {
     for (const auto vbid : shardVbIds[shardId]) {
         const auto start = std::chrono::steady_clock::now();
         auto vb = store.getVBucket(vbid);
+        if (!vb) {
+            continue;
+        }
         auto& epVb = dynamic_cast<EPVBucket&>(*vb);
 
         auto storageCB = std::make_shared<LoadSyncWrites>(epVb);