http: don't clobber the receive buffer for timecond

lib/http.c

@@ -1695,9 +1695,10 @@ CURLcode Curl_add_timecondition(struct Curl_easy *data,
                                 Curl_send_buffer *req_buffer)
 {
   const struct tm *tm;
-  char *buf = data->state.buffer;
   struct tm keeptime;
   CURLcode result;
+  char datestr[80];
+  const char *condp;
 
   if(data->set.timecondition == CURL_TIMECOND_NONE)
     /* no condition was asked for */
@@ -1710,6 +1711,21 @@ CURLcode Curl_add_timecondition(struct Curl_easy *data,
   }
   tm = &keeptime;
 
+  switch(data->set.timecondition) {
+  default:
+    return CURLE_BAD_FUNCTION_ARGUMENT;
+
+  case CURL_TIMECOND_IFMODSINCE:
+    condp = "If-Modified-Since";
+    break;
+  case CURL_TIMECOND_IFUNMODSINCE:
+    condp = "If-Unmodified-Since";
+    break;
+  case CURL_TIMECOND_LASTMOD:
+    condp = "Last-Modified";
+    break;
+  }
+
   /* The If-Modified-Since header family should have their times set in
    * GMT as RFC2616 defines: "All HTTP date/time stamps MUST be
    * represented in Greenwich Mean Time (GMT), without exception. For the
@@ -1718,8 +1734,9 @@ CURLcode Curl_add_timecondition(struct Curl_easy *data,
    */
 
   /* format: "Tue, 15 Nov 1994 12:45:26 GMT" */
-  snprintf(buf, BUFSIZE-1,
-           "%s, %02d %s %4d %02d:%02d:%02d GMT",
+  snprintf(datestr, sizeof(datestr),
+           "%s: %s, %02d %s %4d %02d:%02d:%02d GMT\r\n",
+           condp,
            Curl_wkday[tm->tm_wday?tm->tm_wday-1:6],
            tm->tm_mday,
            Curl_month[tm->tm_mon],
@@ -1728,22 +1745,7 @@ CURLcode Curl_add_timecondition(struct Curl_easy *data,
            tm->tm_min,
            tm->tm_sec);
 
-  switch(data->set.timecondition) {
-  default:
-    break;
-  case CURL_TIMECOND_IFMODSINCE:
-    result = Curl_add_bufferf(req_buffer,
-                              "If-Modified-Since: %s\r\n", buf);
-    break;
-  case CURL_TIMECOND_IFUNMODSINCE:
-    result = Curl_add_bufferf(req_buffer,
-                              "If-Unmodified-Since: %s\r\n", buf);
-    break;
-  case CURL_TIMECOND_LASTMOD:
-    result = Curl_add_bufferf(req_buffer,
-                              "Last-Modified: %s\r\n", buf);
-    break;
-  }
+  result = Curl_add_buffer(req_buffer, datestr, strlen(datestr));
 
   return result;
 }