Summary:
It seems clear to me that the variable is initialized before line 492, but it wasn't clear to UBSAN. The failure was:
```
In file included from ./env/io_posix.h:14:0,
from env/env_posix.cc:44:
./include/rocksdb/env.h: In member function ‘virtual rocksdb::Status rocksdb::{anonymous}::PosixEnv::NewMemoryMappedFileBuffer(const string&, std::unique_ptr<rocksdb::MemoryMappedFileBuffer>*)’:
./include/rocksdb/env.h:822:36: error: ‘base’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
: base(_base), length(_length) {}
^
env/env_posix.cc:482:11: note: ‘base’ was declared here
void* base;
```
We can just initialize to nullptr to keep UBSAN happy.
Closes https://github.com/facebook/rocksdb/pull/3770
Differential Revision: D7756287
Pulled By: ajkr
fbshipit-source-id: 0f2efb9594e2d3a30706a4ca7e1d4a6328031bf2
dfc61e7c24