修复ffmpeg日志保存目录相关bug

每次调用addFFmpegSource/getSnap接口时，都可能导致篡改全局配置变量

server/Process.cpp

@@ -108,7 +108,7 @@ static int cloneFunc(void *ptr) {
 
 #endif
 
-void Process::run(const string &cmd, string &log_file) {
+void Process::run(const string &cmd, string log_file) {
     kill(2000);
 #ifdef _WIN32
     STARTUPINFO si = { 0 };

server/Process.h

@@ -26,7 +26,7 @@ class Process {
 public:
     Process();
     ~Process();
-    void run(const std::string &cmd, std::string &log_file);
+    void run(const std::string &cmd, std::string log_file);
     void kill(int max_delay,bool force = false);
     bool wait(bool block = true);
     int exit_code();