fix use-after-free error involving a temporary string (#4240)

Summary:
In the current code, `error_msg` is pointing to the inner buffer of a temporary std::string object. When `error_msg` is used to construct the error message, that array is already released. This PR will fix this bug by copying the string to a local variable.
Fixes https://github.com/facebook/rocksdb/issues/4239
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4240

Differential Revision: D9204334

Pulled By: miasantreble

fbshipit-source-id: 0ac599e166ae0a4ec413e32d8b8853d7c5fba878

db/db_impl_open.cc

@@ -361,7 +361,7 @@ Status DBImpl::Recover(
       s = env_->NewRandomAccessFile(IdentityFileName(dbname_), &idfile,
                                     customized_env);
       if (!s.ok()) {
-        const char* error_msg = s.ToString().c_str();
+        std::string error_str = s.ToString();
         // Check if unsupported Direct I/O is the root cause
         customized_env.use_direct_reads = false;
         s = env_->NewRandomAccessFile(IdentityFileName(dbname_), &idfile,
@@ -371,7 +371,7 @@ Status DBImpl::Recover(
               "Direct I/O is not supported by the specified DB.");
         } else {
           return Status::InvalidArgument(
-              "Found options incompatible with filesystem", error_msg);
+              "Found options incompatible with filesystem", error_str.c_str());
         }
       }
     }