epee: Don't set log file name when process path name isn't found

If process path name isn't found, then leave log file name blank.

This also applies if a process name is found, but it's blank after
removing a trailing dot extension.

contrib/epee/include/misc_log_ex.h

@@ -861,6 +861,7 @@ namespace log_space
       std::string::size_type a = m_default_log_file.rfind('.');
       if ( a != std::string::npos )
         m_default_log_file.erase( a, m_default_log_file.size());
+      if ( ! m_default_log_file.empty() )
         m_default_log_file += ".log";
 
       return true;