The vector rep implementation was segfaulting because of incorrect initialization of vector.

Summary:
The constructor for Vector memtable has a parameter called 'count'
that specifies the capacity of the vector to be reserved at allocation
time. It was incorrectly used to initialize the size of the vector.

Test Plan: Enhanced db_test.

Reviewers: haobo, xjin, emayanke

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13083

db/db_test.cc

@@ -335,7 +335,7 @@ class DBTest {
         options.memtable_factory.reset(new UnsortedRepFactory);
         break;
       case kVectorRep:
-        options.memtable_factory.reset(new VectorRepFactory);
+        options.memtable_factory.reset(new VectorRepFactory(100));
         break;
       case kUniversalCompaction:
         options.compaction_style = kCompactionStyleUniversal;

include/rocksdb/memtablerep.h

@@ -152,7 +152,7 @@ class MemTableRepFactory {
 // Parameters:
 //   count: Passed to the constructor of the underlying std::vector of each
 //     VectorRep. On initialization, the underlying array will be at least count
-//     size.
+//     bytes reserved for usage.
 class VectorRepFactory : public MemTableRepFactory {
   const size_t count_;
 public:

util/vectorrep.cc

@@ -123,10 +123,10 @@ size_t VectorRep::ApproximateMemoryUsage() {
 }
 
 VectorRep::VectorRep(const KeyComparator& compare, Arena* arena, size_t count)
-  : bucket_(new Bucket(count)),
+  : bucket_(new Bucket()),
     immutable_(false),
     sorted_(false),
-    compare_(compare) { }
+    compare_(compare) { bucket_.get()->reserve(count); }
 
 VectorRep::Iterator::Iterator(class VectorRep* vrep,
                    std::shared_ptr<std::vector<const char*>> bucket,