去除编译警告，修复bug

server/System.cpp

@@ -148,7 +148,7 @@ void System::systemSetup(){
         ss << "## exe:       " << exeName() << endl;
         ss << "## signal:    " << sig << endl;
         ss << "## stack:     " << endl;
-        for (int i = 0; i < stack.size(); ++i) {
+        for (size_t i = 0; i < stack.size(); ++i) {
             ss << "[" << i << "]: ";
             for (auto &str : stack[i]){
                 ss << str << endl;

server/WebApi.cpp

@@ -187,7 +187,7 @@ static inline void addHttpListener(){
                                                 const HttpBody::Ptr &body) {
 
                 //body默认为空
-                size_t size = 0;
+                ssize_t size = 0;
                 if (body && body->remainSize()) {
                     //有body，获取body大小
                     size = body->remainSize();