mlog: don't remove old logs if we failed to rename the current file

Coverity 188348

contrib/epee/src/mlog.cpp

@@ -137,7 +137,12 @@ void mlog_configure(const std::string &filename_base, bool console, const std::s
   el::Loggers::addFlag(el::LoggingFlag::StrictLogFileSizeCheck);
   el::Helpers::installPreRollOutCallback([filename_base, max_log_files](const char *name, size_t){
     std::string rname = generate_log_filename(filename_base.c_str());
-    rename(name, rname.c_str());
+    int ret = rename(name, rname.c_str());
+    if (ret < 0)
+    {
+      // can't log a failure, but don't do the file removal below
+      return;
+    }
     if (max_log_files != 0)
     {
       std::vector<boost::filesystem::path> found_files;