logging: Fix potential use-after-free in LogPrintStr(...)

[practicalswift]

Fix potential use-after-free in LogPrintStr(...).

freopen(…) frees m_fileout.

[laanwj]

Feel free to take over https://github.com/laanwj/bitcoin/tree/2018_05_logprintf_remove_return_value, which removes the unused and incorrect return value from LogPrintstr and has this commit rebased on top.

[practicalswift]

@laanwj Nice cleanup! Added that commit to this PR.

[laanwj]

utACK 0bd4cd3

[promag]

utACK 0bd4cd3.

[ajtowns]

I don't think this patch is really correct -- in the usual failure case where the old file handle is closed, but a new file can't be (re)opened (due to permissions, eg), we'll silently drop the current log message, then start writing any future log messages to the m_msgs_before_open buffer, which will never be cleared. Am I missing something?

If I'm not, seems like better behaviour might be something like:

     if (m_reopen_file) {
          m_reopen_file = false;
          FILE* new_fileout = fopen(m_file_path, "a");
          if (!new_fileout) {
               // release m_file_mutex first
               LogPrintf("Failed to reopen log file %s\n", m_file_path);
          } else {
               fflush(m_fileout);
               fclose(m_fileout);
               m_fileout = new_fileout;
          }
     }

(Would it make sense to do StartShutdown() if logging stops working?)