fix the non initialized bug in StderrLogger. (facebook#12839)

Summary:
This PR is intended to fix a potential uninitialized variable bug.

Fixes facebook#12837

Pull Request resolved: facebook#12839

Reviewed By: ajkr

Differential Revision: D59398888

Pulled By: cbi42

fbshipit-source-id: 337391d7c1e73c0ff61797f88fbb4a8379500211

util/stderr_logger.h

@@ -17,7 +17,7 @@ namespace ROCKSDB_NAMESPACE {
 class StderrLogger : public Logger {
  public:
   explicit StderrLogger(const InfoLogLevel log_level = InfoLogLevel::INFO_LEVEL)
-      : Logger(log_level), log_prefix(nullptr) {}
+      : Logger(log_level), log_prefix(nullptr), log_prefix_len(0) {}
   explicit StderrLogger(const InfoLogLevel log_level, const std::string prefix)
       : Logger(log_level),
         log_prefix(strdup(prefix.c_str())),