http: pass correct header size to debug callback for chunked post

... when the chunked framing was added, the size of the "body part" of
the data was calculated wrongly so the debug callback would get told a
header chunk a few bytes too big that would also contain the first few
bytes of the request body.

Reported-by: Dirk Wetter
Ref: #6144
Closes #6147

lib/http.c

@@ -2873,20 +2873,24 @@ CURLcode Curl_http(struct connectdata *conn, bool *done)
         }
         else {
           if(postsize) {
+            char chunk[16];
             /* Append the POST data chunky-style */
-            result = Curl_dyn_addf(&req, "%x\r\n", (int)postsize);
+            msnprintf(chunk, sizeof(chunk), "%x\r\n", (int)postsize);
+            result = Curl_dyn_add(&req, chunk);
             if(!result) {
+              included_body = postsize + strlen(chunk);
               result = Curl_dyn_addn(&req, data->set.postfields,
                                      (size_t)postsize);
               if(!result)
                 result = Curl_dyn_add(&req, "\r\n");
-              included_body = postsize + 2;
+              included_body += 2;
             }
           }
-          if(!result)
+          if(!result) {
             result = Curl_dyn_add(&req, "\x30\x0d\x0a\x0d\x0a");
-          /* 0  CR  LF  CR  LF */
-          included_body += 5;
+            /* 0  CR  LF  CR  LF */
+            included_body += 5;
+          }
         }
         if(result)
           return result;