memcached crash, assert in CouchKVStore::saveDocs

Running RebalanceCombo tests that do writes during incremental rebalance-in, followed by verification.

Rebalance failed and then memcached aborted with bt:

Thread 1 (Thread 0x4682c940 (LWP 16053)):
#0 0x00002ae3ad47d265 in raise () from /lib64/libc.so.6
#1 0x00002ae3ad47ed10 in abort () from /lib64/libc.so.6
#2 0x00002ae3ad4766e6 in __assert_fail () from /lib64/libc.so.6
#3 0x00002aaaaadc4611 in CouchKVStore::saveDocs (this=0xe23a000, vbid=11, rev=2, docs=0x248987a0, docinfos=0x32d8afa0, docCount=13) at couch-kvstore/couch-kvstore.cc:1154
#4 0x00002aaaaadc47b8 in CouchKVStore::commit2couchstore (this=0xe23a000) at couch-kvstore/couch-kvstore.cc:1082
#5 0x00002aaaaadc49b2 in CouchKVStore::commit (this=0xe23a000) at couch-kvstore/couch-kvstore.cc:620
#6 0x00002aaaaad3ac62 in TransactionContext::commit (this=0xe252360) at ep.cc:2676
#7 0x00002aaaaad3b1dd in EventuallyPersistentStore::flushSome (this=0xe252000, q=0xe2522c0, rejectQueue=0x20852780) at ep.cc:1974
#8 0x00002aaaaad650e4 in Flusher::doFlush (this=0xe250090) at flusher.cc:244
--Type <return> to continue, or q <return> to quit--
#9 0x00002aaaaad65cb5 in Flusher::step (this=0xe250090, d=..., tid=std::tr1::shared_ptr (count 5) 0xe234500) at flusher.cc:158
#10 0x00002aaaaad66330 in FlusherStepper::callback (this=<value optimized out>, d=<value optimized out>, t=<value optimized out>) at flusher.cc:23
#11 0x00002aaaaad2e7bf in Task::run(Dispatcher&, std::tr1::shared_ptr<Task>) () from /opt/couchbase/lib/memcached/ep.so
#12 0x00002aaaaad2d670 in Dispatcher::run (this=0xe216c40) at dispatcher.cc:123
#13 0x00002aaaaad2defb in launch_dispatcher_thread (arg=<value optimized out>) at dispatcher.cc:28
#14 0x00002ae3ad23873d in start_thread () from /lib64/libpthread.so.0
#15 0x00002ae3ad5214bd in clone () from /lib64/libc.so.6