http: fix the max header length detection logic

Previously, it would only check for max length if the existing alloc
buffer was to small to fit it, which often would make the header still
get used.

Reported-by: Guido Berhoerster
Bug: https://curl.haxx.se/mail/lib-2018-02/0056.html

Closes #2315

lib/http.c

@@ -2880,20 +2880,19 @@ static CURLcode header_append(struct Curl_easy *data,
                               struct SingleRequest *k,
                               size_t length)
 {
-  if(k->hbuflen + length >= data->state.headersize) {
+  size_t newsize = k->hbuflen + length;
+  if(newsize > CURL_MAX_HTTP_HEADER) {
+    /* The reason to have a max limit for this is to avoid the risk of a bad
+       server feeding libcurl with a never-ending header that will cause
+       reallocs infinitely */
+    failf(data, "Rejected %zd bytes header (max is %d)!", newsize,
+          CURL_MAX_HTTP_HEADER);
+    return CURLE_OUT_OF_MEMORY;
+  }
+  if(newsize >= data->state.headersize) {
     /* We enlarge the header buffer as it is too small */
     char *newbuff;
     size_t hbufp_index;
-    size_t newsize;
-
-    if(k->hbuflen + length > CURL_MAX_HTTP_HEADER) {
-      /* The reason to have a max limit for this is to avoid the risk of a bad
-         server feeding libcurl with a never-ending header that will cause
-         reallocs infinitely */
-      failf(data, "Avoided giant realloc for header (max is %d)!",
-            CURL_MAX_HTTP_HEADER);
-      return CURLE_OUT_OF_MEMORY;
-    }
 
     newsize = CURLMAX((k->hbuflen + length) * 3 / 2, data->state.headersize*2);
     hbufp_index = k->hbufp - data->state.headerbuff;