Get unique_ptr to use delete[] for char[] in DumpMallocStats

Summary:
Avoid mismatched free() / delete / delete [] in DumpMallocStats
Closes https://github.com/facebook/rocksdb/pull/1927

Differential Revision: D4622045

Pulled By: siying

fbshipit-source-id: 1131b30

db/db_impl.cc

@@ -604,7 +604,7 @@ static void DumpMallocStats(std::string* stats) {
 #ifdef ROCKSDB_JEMALLOC
   MallocStatus mstat;
   const uint kMallocStatusLen = 1000000;
-  std::unique_ptr<char> buf{new char[kMallocStatusLen + 1]};
+  std::unique_ptr<char[]> buf{new char[kMallocStatusLen + 1]};
   mstat.cur = buf.get();
   mstat.end = buf.get() + kMallocStatusLen;
   malloc_stats_print(GetJemallocStatus, &mstat, "");