improve when content-len is 0 (#336)

thirdparty/cinatra/cinatra/coro_http_client.hpp

@@ -317,9 +317,11 @@ class coro_http_client {
   }
 #endif
 
-  // it may return nullptr, because maybe no body_ allocated, the read_buf_ may
-  // read all head and body, at that time the body_ is nullptr.
+  // return body_, the user will own body's lifetime.
   std::unique_ptr<char[]> release_buf() {
+    if (body_.size() == 0) {
+      return nullptr;
+    }
     return std::unique_ptr<char[]>(body_.release());
   }
 
@@ -1228,10 +1230,12 @@ class coro_http_client {
       if ((size_t)parser.body_len() <= read_buf_.size()) {
         // Now get entire content, additional data will discard.
         // copy body.
-        body_.init(content_len);
-        auto data_ptr = asio::buffer_cast<const char *>(read_buf_.data());
-        memcpy(body_.data(), data_ptr, content_len);
-        read_buf_.consume(read_buf_.size());
+        if (content_len > 0) {
+          body_.init(content_len);
+          auto data_ptr = asio::buffer_cast<const char *>(read_buf_.data());
+          memcpy(body_.data(), data_ptr, content_len);
+          read_buf_.consume(read_buf_.size());
+        }
         co_await handle_entire_content(data, content_len, is_ranges, ctx);
         break;
       }