initialize local variable for UBSAN in PosixEnv function

Summary:
this is a repeat commit of a8a28da215, which got reverted together with 6afe22db2e, but forgotten about when that commit was un-reverted in 46152d53bf.
Closes https://github.com/facebook/rocksdb/pull/3796

Differential Revision: D7826077

Pulled By: ajkr

fbshipit-source-id: edb22375da56e2feda50c5b35f942f4d2d52b19c

env/env_posix.cc

@@ -479,7 +479,7 @@ class PosixEnv : public Env {
     if (status.ok()) {
       status = GetFileSize(fname, &size);
     }
-    void* base;
+    void* base = nullptr;
     if (status.ok()) {
       base = mmap(nullptr, static_cast<size_t>(size), PROT_READ | PROT_WRITE,
                   MAP_SHARED, fd, 0);